I would take the files out to an editor ( Audacity is Free) and save the files as 16 bit, then reload to a 16 bit project.
I have been in your position, with projects that got bigger than expected in my case.
You can mix the parts you have, or some combo of them and create a (Stereo) stem, free the tracks in the stem then move this stereo mix back to a pair of tracks and record to this on the tracks you've freed up. Depending on the song and the nature of the stem, this can work effectively. The song Razor recently posted "Let Them Eat Cake" was built that way, around a stem.
Exporting and then Saving as 16 bit files would be easy and sure to work in your case. I wouldn't throw the tracks away, if you like them.
_________________ Byron
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