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After more than 13 years, and tens of thousands of questions, Moz Q&A closed on 12th December 2024. Whilst we’re not completely removing the content - many posts will still be possible to view - we have locked both new posts and new replies. More details here.
I would say that you want to keep it n the same domain. If you use an m. version of the site, you can keep it on the same sub-domain.
A second option would be to use the same page structure and use 301 redirects so you can move across the link juice from all the back-links.
The obvious first choice is to redesign and slowly integrate however this is not an option here.
Damn it, nice answer, you beat me to the punch