Hello.
My question is: am I able to take other account's name if I prove that I own the other account (and therefore the name) or the person agrees to do that.

And thus, what happens to the name of the account that 'loses' its old name?
So if i want to namechange to, let's say, Vatari (the account isn't yet inactive for 3 years and I own the account), and let's say I'm allowed to do so. Will the current Vatari account be also namechanged to Father or it will become just Vatari_old?

You have to pay for both name changes typically

you have to pay for both namechanges if you want to switch them
but i am not sure how much you pay to namechange just one

Thank you, that's a useful info.

So is it impossible to pay for a single namechange (without switching two names) and get Vatari and Vatari_old names?