One thing I've tried in the past is to do extreme voicing on a set of hammers about to be removed for replacement. Another is to try voicing the very worst note on a customer's piano, one which sticks out very badly.
I've had a good time on a grand (sometimes a good grand) going through the treble and shift voicing wanting to make them even. I find a note brighter than the neighbors, then using a mute and/or fingers I listen to each string, to see if one is brighter or falser than the others. If so, I try working only on it, using the invaluable Hart voicing tool (brass chopstick with removable needle.) I only use one needle, always, no longer than about 1/3". If I find one string I want to soften or take the edge off, using a flashlight, I place the voicing needle about 1/4" in front of the string groove, angled about 45 degrees, and press it in. Listen. Repeat if need be.
If all three strings are equally harsh I do this voicing to all three, then listen to see if any adjustments need to be made to others to get them all in line. If a note is very harsh and the 45 degrees, getting the needle in under the start of the string grooves, putting some air there, isn't enough I put the needle vertically over the center of the string grooves, and press it into each one.
With one needle so short I'm not doing any permanent damage to the structure of the hammer. And by not jabbing forcefully or using a longer needle I'm not risking breaking it off.
For the extreme treble if one or two notes stick out like broken glass up where there is usually a ton of juice turning the hammer to wood, and very little felt to spare, I mark the keysticks with chalk to show which are really bad, then I pull the action onto my lap, and put just one or two drops of vodka right into the string grooves. Not enough to soak deep into the shoulders, just a local treatment where the hammers hit the strings. Then do a little bit more tuning to give the alcohol time to settle in, and check for evenness up there. This is for more basic pianos in rougher condition, not for concert instruments, which should have been set up better to begin with.
Pianos which are really awful, like over-voiced and beaten in Korean instruments with rock-hard hammers, present you with the chance to improve your skills, assuming the customer agrees with you. Never assume that you know best when it comes to voicing. Some people adore what makes you cringe. But if you come across a piano with such tin can broken glass edgy rock hard hammers that you wince, and the customer does, too, you can attempt a quick remediation for very little time or money, assuming you keep a strong discipline and avoid extreme measures which might trash a set of (already pretty trashy) hammers. Take off the fallboard, putting back in the keyblocks so that the action is properly placed, then take a piece of chalk and draw a line along the keysticks (just inside where the fallboard would be) in sections needing help. Leave any medium acceptable areas blank, and put an extra mark on the keystick of any note sticking out very badly.
Pull the action onto your lap. If the string grooves are very long, just nip off the very ends of them with a sandpaper hammer file, restoring a little bit more of an oval shape, doing them uniformly. Do not shorten the hammers by filing all the way across. This is emergency intervention, not a full reshaping of the hammers, and for a piano with hammers this worn, there will be sections which have no felt to lose.
Then, using a dropper bottle with "vodka" (which is 100 proof, or 50% water -- I make it by mixing 190 proof ethanol from the liquor store with 50% tap water), use just a few drops per hammer, across all your marked sections, right in the string grooves. Then give a little bit more to the offending notes which you gave a second mark. The place will smell like a bar room, but pure ethanol from a liquor store is very safe to breathe, for anyone. (I never touch "denatured", there's no need to use anything in the slightest toxic.) For the offending notes needing more, you can add a little bit more alcohol, but be sure not to wet the shoulders any lower than the area between 11 a.m. and 1 p.m. on the clock face, so to speak.
Put the action back in. It should be a pleasant surprise for both you and the customer. Do some tuning while it sorts itself out, then see which notes are not even with the rest. Wipe off your chalk marks with a barely damp shop towel, and make a new set for notes which are still too harsh. Also use the shift pedal, and mark, a different place on the keystick, any which need voicing down, which will need to be done with the chopstick tool in between the grooves, after checking to see where the hammer meets the strings with the left pedal down. When evening up the first pass, use needles except in the extremely high treble, instead of more alcohol.
If a note has gotten too dull compared to the rest, there's a little trick which helps some, though it is usually fairly temporary. Having found the dull note, press down the strings (all 3 wires) about half way back, or as far as you can reach. If the wire is nice and new and clean, put something like a felt mute or a piece of leather on it so your hand sweat won't rust it. Pressing hard on the wire with a finger of your right hand, pound the note repeatedly and quickly with your left. Then take your hand away and see if the brightness is a better match with the others. Since the pressed wires give the hammer no chance to push them away, they press harder into the string grooves than if they were normally played. The sweet part of this is that you are altering only the voicing in the rest position, not the soft pedal voicing. It also is fairly temporary, but it can get all the notes to line up nicely at the time.
There are more extreme things to do to pianos where the voicing is really terrible, and the hammers have little or nothing to lose, but best to get on top of this level of work first. So, try a note or two when they are especially awful, use any opportunities to try the vodka thing on really awful pianos, given permission, and since you are inexperienced, and the procedures take so little time, an extremely low fee is only reasonable. Like $10. I also like to tell customers that I prefer to do voicing piecemeal, taking care of the worst, and doing a little bit when I see the piano later instead of doing a major effort all at once.
If you feel like taking on a project piano (upright or grand) which you can buy for free, for just the moving cost, or for almost free, you can start slowly with it, get the hang of the modest procedures, and then move to the extreme ones, before removing and replacing the hammers. For a freeby like this, which also might need other things, very inexpensive hammers would work fine.
It helps if you are able to play the piano a little bit yourself. You can see how it feels to play with the hammers in different conditions, and which parts of the scale need more or less intervention.
The same joke reappears every time ... I can only say, I never drink the piano alcohol. It's also good for softening glue joints (like when needing to replace a broken hammer shank), or to remove keybushings.