1 - when you install a extension pack with extension manager the voices will be in the first LSB empty (avaiable).
2- - to preserve old registration on sty and songs, you must see what LSB were used and for what.
3- in your case, you must check which is the LSB of the original pack before you create a new pack. On new pack ck set manually the LSB to that number (or numbers)
note: if it is the unique pack installed, usually the original and new pack will be installed with the same LSBs (as told in item 1 above).
MSB always will be the same 062 or 063 (voices e drums for extension packs). Perhaps you will need take attention with the PC number (Program change number). They must be the same to the voices. Check them before create the new pack and set it manually if necessary.
For MSB, LSB, PC issues: imagine a building. LSB is the floor, MSB is the appartment inside that floor, PC is the room inside the appartment.
You have 128 floors, with 128 appartments, with 128 rooms.
Extension packs wil be always on appartments (MSB) 62 e 63 of each floor (LSB). The voices will be insde of rooms (PC)
To save internal memory, use flash USB memory. Only extension packs must be inside USER area of internal memory.
SX600 works as computer: directories and files names. You can create as many as you want on USB flash memory, and save all things there.
Example: directory song, you have song files. Directory pad, you have multipads files. Directory style, you have sty files .....