From what I've seen, generally old forts will work in the new versions. The main downside is they may bring some of the old version's problems with them, but if your alternative is continuing to use the old version, then that doesn't lose you anything. You also get some 'useless' carry-over, like the Armory noble (your existing noble will continue to hold the position, but it will have no function), overstuffed hospitals (until you get rid of what's already in there), and such. My one area of doubt would be the military, since it seemed to get a couple major changes during 31.x so far - it's always come across successfully for me, but I usually have forts jump one or two versions, rather than trying to get a 31.12 fort to run in 31.25 or the like (though it would probably work). My current fort has gone through more versions than most, 31.19 -> .21 -> .22 -> .23 -> .24 -> .25, due to the rapid releases. The biggest single jump was a 31.12->.13 fort in 31.18 - I didn't continue it very long in .18 so it's not a particularly thorough test, but the initial impression was that it came across fine.
The soap fixes (both the crash-from-partial-bars and the job-cancel spam) seem to translate from earlier versions successfully, in my experience. The diagnosis and surgery fixes seemed to also translate.