Hi Serge - just joined the forum today - so I'm not an expert. ;)
But I saw your post, and I've some experience with UNIX/Linux plus a few years with Ubuntu on X86 - but not on PPC.
My feeling would be that if 7.10 (Gutsy) is still working, it's safer to leave it alone. It reached end of life nearly 3 years ago, so I'm surprised if the repository is still offering updates - which repo is it using?
Upgrades to the next version (8.04) were a bit problematic on 7.10, iirc. Ours crashed, had to revert to backups. So I would not risk doing that. An upgrade to an even later version is even less likely to succeed, imho.
So if it ain't broke, better not to risk it.