I think if you try to leave the Insider program, it'll ask you to do that (stay on current build, but don't receive future fast ring builds). Then you could switch to Release Preview.
The only thing I'm not sure about, is whether it'll let you switch to Release Preview immediately (to get app updates), or whether it'll make you wait for a Release Preview OS build to be available (probably 2-3 weeks).
Either way, you can leave Fast ring without losing anything, but you might not get the app updates immediately.