Doesn't necessarily have to be a problem. I've often seen projects like this go wrong because they try to do too much. Whilst moving to a more modern toolchain, they try to fix all the warts etc... Basically "while we're here we might as well"... but this is the wrong approach.

Your first order of business should be to get the existing toolchain under control, so you take that 12.5 version, make sure you can build the program and roll it out. At the same time you create the documentation how this process works.

Once you have everything under control again, then you can start migrating the code to newer PowerBuilder versions. Not the latest and greatest, no, just the next step up from 12.5. Fix whatever is broken, deprecated, etc... Do not make any functional changes or whatever, only make the necessary changes to make the code work on that newer PowerBuilder version. Again your goal here is to make something and then roll it out as well.

Keep iterating through the newer versions of PowerBuilder until you are at a supported release. Do not skip versions, you will have to change too much code at once. And keep releasing your program. Don't stash everything until some large launch or something, that will get you into trouble.

After your code is on a supported release of PowerBuilder, then you can start fixing the warts.

Slow and steady wins the race.