Question Site Migration - Duplicate Content Question

[Remnt]

Hi, so i'm moving my forum to a new URL . I will setting up a 301 redirect once it's ready. Naturally I want to port over alot of the official posts and content I made on the old forum as it's alot. I'm moving for a brand/domain change, but most of the content i'd want to keep.

I think normally this is fine if the old forum wouldn't be available later but the old URL is actually a custom domain which is mapped over the free forum hoster's URL (so hard to explain without showing links but unable to). So even if there's a redirect on 'Old Forum A' - > New Forum B, the old forum will forever exist (unless I manually delete it).

I'd rather not delete the old forum and put a watermark saying "we've moved". Then again, all my traffic/advertising goes to Old Forum A's custom domain (which is why I want to pass all the link juice over). My main goal is not to have duplicate content penalties thrown at me.

Example: Old Forum A pointing to Old Forum A Custom Domain which will be redirected to New Forum B (Old Forum A & it's Custom Domain are both live, there's no redirect inbetween those two).

My question is, if i'm doing a 'change of address' in Google Search Console, will i still get a penalty for duplicate content because Old Forum A would still exist and share the same content?

Any advice on the best procedure for this would be much appreciated.