thank you for your responses.
So i understand better: the new physical volumes should be at the end of the file system, and the heads must move from the beginning to the end.
Effectively the data are already saved, but there is always the risk of a corrupted backup... Or perhaps if I install 1To more, i move the file system to the new, i verify that all is right and I delete the old file system.
The file system is jfs2, and i have verified, i can extend it