You should be able to unfreeze and go back to MIDI.
Here's what I do btw:
- make a copy of the MIDI track
- freeze the copy to generate an audio track
- copy the frozen audio to a new track
- archive the original MIDI track and the frozen copy
I've been burnt too many times by going back to older projects only to find that one of the following has occurred:
- the MIDI synth no longer works, is not available, plays the sound differently, or 'what sound'
- the frozen track cannot be unfrozen
My approach works for me because at least I know I have a permanent copy of the audio the way I heard it when I was composing the MIDI track. I also like to know I have that original MIDI track archived should I need to work on it further. Keeping the frozen track might seem redundant, but I like to have it around just in case.
At the end of the day I trust no synth, no software, no bundle backup or older disks etc. to be able to recreate the audio sound from its components. I always, always render that audio as my ultimate fallback situation.