Thank you Richard for your kind answer.

Yes, the application programs are very different so a new program must be loaded each time a different extension is connected. The application program is permanently stored in the extension EEPROM and copied into the PIC program space when necessary. However, changing the extensions is not so frequent so I don't thing that the durability of the flash EPROM could be affected. Do you have any data about the number of reprogrammations which are accepted for this kind of memory ?

Changing the PIC model is no more possible at this level of the development and even 128K would not be large enough to contain all the possible extension programs.
Moreover, from an evaluation I have done 3 or 4 years ago I remember that the K40 and K42 are not correctly supported by PBP + MPLAB 8. You have to go to MPLAB X with some ennoying issues regarding the debugging at the source level. My current development environment is MPLAB 8 + PBP + PicKit 2 and 3. I also own a Real Ice.

Thank you for your help

MikeBZH