Wow, this is gonna be one long project over xmas. It'll keep me out of trouble lol. But I'm hoping the long term strategy bears fruit
Honestly, have a word with your hosting company to find the simpliest way of doing it which won't damagin your content that does already rank well.
When I moved from SMF Forum software to XenForo EVERYTHING changes, the URLS, profiles, internal links, THE LOT! It was a huge nightmare which from a user point of view was absolutely fine but had me tearing my hair out but sometimes you have to just suck that up because you are doing 'short term pain for long term gain'!
The issue I had was that all the old links in google were now pointing to threads that no longer existed and I'm SURE I had a add some code into my .htaccess to make it so if people hit my site by clicking a link with the prefix https:///themoneyshed.co.uk/oldforum/thread they would get re-directed to /newforum/thread
it's all possible basically!
I totally understand why LARGE sites hate making wholesale changes. the green site forum should have been moved to new forum software years ago but lords know how you deal with a site THAT complex but sometimes I just needs doing to improve user experience or whatever.