I am replying to myself since no one did !
I have started the process. I first replaced eprom 2.09 by MK2 eprom 4.28 which is the last one. It is OK. Everything seems to work. But my own sounds cannot be loaded in this new version. Not a serious issue since the sounds and presets can now be edited easily and quickly on my PC as my Blue Chip AX-100 SB now communicates with my PC.
Only one major issue : I wanted to update the firmware from 4.28 to v. 7.28, so I followed the manual's instructions :
"Press the "PRESET" + "UTILITY" + "EDIT" buttons while switching on the AX 100 MKII. The following appears in the display: "TERRATEC AXON FIRMWARE UPDATE"..."
But unfortunately when I press these 3 buttons and power on the unit this update display NEVER appears...
Is anybody here knowing why
PS: I also replaced the sound card by one which is more compatible with MK2 version (better sounds and faster response but I don't think this has anything to do with the issue