I may have a solution with two separate development chains but both installed (to validate).

For maintening my old 18F2685 projects :
MPLAB 8 (including MPASM) + PBP 3.0 + PicKit2

For developing my new 18F27K40 project :
MPLAB X (including MPASMX) + PBP 3.1 + PicKit 3 + MCSPX (maybe).

I assume that PBP 3.0 and PBP 3.1 live separatly and are seen as two different programs which allows to establish some independant links between MPASM 8 and PBP 3.0 on one side and MPASM X and PBP 3.1 on the other side. Am I right ?

I will try to test this on an other machine today.

MikeBZH