Migrate but keep old database

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Hello guys, I love Duplicator, but the only issue I have is when lets say I need to move site that I made (New site) for a client to (OldSite)
Old site have few post and pages, me or client dont know database name and password.
I d like to move New site to Old site keeping old clients data base and old clients log in and password.
How would I do that?

Thanks in advance!