You're looking at a different chip!
The basic core of the code is probably "ok". But, some differences:
1. Fuse bits will probably be different
2. The analog/digital config bits may be different
3. The module (adc, PWM, etc) may function differently
Any developer who made the 2520 software should have no problem porting to the 25k22. It requires a bit of time, the job doesn't do itself here, but frankly this is not rocket science.
"does anyone have similar issues"? Actually, everyone does. Everyone who ports a project from one chip to another at the very least has to rebuild. But unless you're just using the exact same part with more memory (which generally does require a rebuild), some porting work must be done.
Wait, you have the source code, right? Why are you burning a copy with the write-protect fuses set? Just change the fuse bits to unprotected and rebuild...