If you were rescaling in this situation, presumably to lower the total tension inherent in raising the pitch of the piano by 20¢, the only factor you would have available is string diameter assuming you aren't planning to make a newly designed bridge. Dropping string diameter by half a gauge per note for the plain wire would almost certainly drop the tension enough, maybe more than enough. Someone with scaling software could provide that answer.
Break point percentage would stay pretty much exactly the same, as the relationship between strength, diameter and tension is in sync (not counting the small difference due to work hardening during the drawing process). The sound would be a wee bit weaker - less vibrational energy.
There was a strong tendency in the second half of the 19th century and early 20th for piano technicians to restring with heavier gauges, going up half to a full gauge for all the plain wire. (How do I know this? A couple piano technical manuals of the late 19th century specifically urge readers not to do this. They must have been reacting to common practice.)
The aim of increasing diameters was to increase tension and thereby make the pianos "louder and more brilliant," in keeping with what was done throughout the century by manufacturers as stronger wire became available. Unfortunately, many of these pianos were not strong enough to resist the additional tension, so we have a lot of twisted square piano cases and raised grand cheeks as a result.
I am bringing this up as a warning not to rely on existing string gauges of older pianos if they have been restrung, especially if they are as early as the mid 19th century. Typical top strings would have been 11 or even 10 gauge.
Best,
Fred Sturm
www.artoftuning.com
http://fredsturm.net"Tradition is not the worship of ashes, but the preservation of fire." -Gustav Mahler