I forgot to write that information. The whole action took us three weeks.
Two days of taking the cassettes out and two days of taking them back in.

Yes, each cassette weighs 300 Kg but not all of them had to be taken out.
There are 4 rows of cassettes. Only the two outer rows had to be taken out to reach the wall. For the two rows in the middle it was enough to push three cassettes together of each end. Same for the second level.
Could I explain it better now? Let me know if I shall explain it in a different way, like, on a photo with drawings. I'm happy to explain 😌