Takeover exicisting system => upload

Hi Community, as a newby i'm slowly learning te steps in the world of KNX.

I finished my basic course and want to step up a little bit.

I recently came across a new client who asked me if I would be interested in taking over an excisting expanded house system because the installer is going on retreat.

I receved te programmation of the system from the installer and want to make a connection localy.

Still there I'm not so sure about myself. If I make connection with the system and start making corrections in the modules, I have to download it to some modules.

What if I received an incomplete programmation, after downloading,  will excisting modules/groupadresses.individual adresses be erased?

I was wondering, is there in any way a possibility to connect with the system and first of all make a full upload of the system,modules, groupadresses and individual adresses?

I would be more sure of myself, i don't want to explain to the client that after 1 setting, half of the building is not working anymore.

Thank for any helpfull hints or trics.