I made the same transition a couple of years back.

PBP 2.47 supports the 18F4620 but has some bank switching issues that may affect you. PBP 2.60+ is worth the upgrade because you get signed LONGs which are very handy for high precision arithmetic and saves a lot of checking to make sure a PIC16F877 results never goes negative.

You will need to open the MPASM file P18F4620.inc and search for the first occurrence of the word DEBUG. Comment this out by placing
;
in the first column followed by a space. Otherwise the PBP reserved word DEBUG gets confused with the MPASM reserved word DEBUG and everything falls apart.

HTH
BrianT