So if anybody gets stucked in this one - here's what i've done:
I put my kmeaw 3.55 CFW on USB, as misiozol told me; and my old 2,5'' hdd into the ps3 and turned it on!
The system asked me to prepare removable storage with 3.55 update or higher...confirmed,and it started to format my HDD,and installed my CFW!
When it started up,and all set up, i swapped the HDD again,and it worked like a charm! no lost data or save game! the only thing to do is now to find the downloaded data and delete it... Thanks again to all participants!