The question is simple: when I change the wallet.dat file to an other one, what was synced already with this client, it will save from the start or from the last point of sync? Any advise can help, thanks.
I don't really get what you mean by that question.
If you're talking about how the client will rescan, it will reindex the blockchain when you enabled prune if you replace the wallet.dat or import any keys. I don't think it requires a reindex if your wallet.dat is last synchronized to the point where the block is still within the pruning range*. The client will rescan from the point where it is last synchronized until.
Eg. If I replace my wallet.dat on 23/8/2017, and I put it back on 27/8/2017, the client will rescan the wallet from 23/8/2017.
The blockchain files itself is not affected.
*I need clarification on that, I haven't tried it myself.