Adding new levels “in between” / upgrading top level

Hello everyone!
I need to change our current membership plans by adding some levels “in between” and I am not sure how to go about it. When I was setting up our membership site 3 years ago I didn’t foresee this. We have grown and have much more content now, meaning we need more memberships plans.

I used only level 1 and level 2 back then… Now I would need to add 2 more levels, but keeping the current level 2 subscribers with the same, top-level access.

Any ideas what would be the best way to do it?
Is there any way to change all level 2 subscribers to level 4 but without any issues with demoting, cancellations later? I can already imagine this being a nightmare.
Using custom capabilities?

Any suggestions would be greatly appreciated