@kbmonkey, MTS' official position is that if you don't subscribe to a channel you won't be able to play previously recorded content. Each program is validated when you start playing it.
This is definitely true with "temporary" preview channels like 40, 41, etc. When the channel is deleted, you can't play old recordings. If you had recordings on channel 40 and it later comes back you can then play the recordings (I think this is true even if channel 40 is actually a different network).
I was playing around with this on regular themes and it doesn't seem to be consistent. In most cases if you record a program and switch themes, you can't play the old program if you're not subscribed to the channel. There have been a few cases where it would play a program recorded a week ago but wouldn't play a program recorded 5 minutes before the theme change. Both programs were on the same channel.
With regular themes, it's easy to re-subscribe to the theme using the on-line tool if you can't play the old recordings. Move Central/HBO requires a minimum 30 subscription.