It should be a straight drop-in replacement, all the Minimigs I've seen have the PIC seated in a socket. The same bootloader we use on the 18F252 should work on the 18F46K20, so users should still be able to upgrade over serial. For the faster SPI transfers, you will still need to do the little solder bridge mod that you do with the ARM controller.
Sounds good, as I would need want to get my Minimig that is still using its PIC upgraded to 4MB anyway I am sure Amigakit could do the SPI mod at the same time for me and yes the PIC chip on both my Minimigs is/was socketed (one was upgraded to have the ARM addon board).
I will look forward to hearing about your progress with the new PIC chip and upgrading my first 2MB Minimig then your new PIC firmware is ready for end user.
Swapping out the PIC chips is no harder than doing a traditional kickstart upgrade on a classic Amiga so I am sure most users should be up to the job if they take things slowly and carefully, I found upgrading to the ARM addon easier than fitting many classic Amiga hardware addons