looking for a PIC

[languer]

We've tried the following with some "limited" success before as arguments for FCC: "Simple component change for obsolescence retains equivalent functionality does not require re-testing". It seems you're trying a similar approach; however, the FCC will ultimately decide what is acceptable or not (which will be expensive regardless). So if you're making the "call" whether or not to inform the FCC about the change; then the following two are equivalent (we've used them with equal degree of "success"):
-> Replacing a component with and equivalent component as long as the clocks used and/or generated by the two do not change.
-> Replacing a component with an equivalent daughter card (component built-in into a small PCB to provide proper pinout and voltage conversion) as long as the clocks used and/or generated by the two do not change.

And yes I do agree with Robert, using a different MCU could likely require approval - since the method they generate the internal clocks are likely to be different. But I again, it would be your "call" whether or not to inform the agency.