I was able to do this when I started a forum earlier this year. I started it out as a general discussion forum but after much thought, I decided I wanted to take on a promotion forum as an owner.
It was quite easy to do by keeping what we already had and moving it into a general area and then adding the other topics on as well, the switch over went smoothly with no issues which I was happy about.
I took my webmaster forum and turned it into an admin forum.
I feel like that was a good pivot as most of the members on the forum were forum owners.
I did give the option of account removal to whoever didn't want to pivot. No one asked to be removed, so I'm guessing they were fine with the pivot.
I've seen extreme pivots in forum niches where something like a Movies and TV forum turned into a business and money forum. I remember those didn't go well and I could understand that since the niche exchange is so diverse from one another.
I once created an insurance forum, I did not switch the niche but added a couple of more categories and changed the domain form insurance niche to investment niche