I would advise against a rebrand until after the merge has already been done. I did a rebrand, and I'm kinda regretting parts of the rebrand. What I did in the rebrand is refresh the database completely. Killing all SEO. So, you BEST have all the links retained. Anytime.
The old website had like thousands and thousands of inbound traffic. Without any pagerank, you can never recover.
Just merge the database into Admin Junkies first, then consider the rebrand. That way, all requests from domain "A" to domain "B" is maintained. I did this in the transition from PS4Forum to PS5Forum. PS4Forum was on vB4, and I converted the database into xenForo, then put redirects in place so the parking of PS5Forum was strong enough.
With DestroyRepeat, my original plan was to keep everything the way it was, without any issue. But I discovered exploits and I'd known about it for a while now. I was planning to refresh the database, but keep the old links, but what I did is not maintained the old ID's because I'd need to remember the password for MY username/pass. Or something else. I forget which. Either way, that was a bad move, so all those thousands of inbound traffic? Gone.
So, y'all need a plan.