Sort of just went through something similar. I purchased a 7night Cat8 (now post-merger cat5) package on August 17th 2018 right before the dead period for the merger, and attached it to a hotel. This past few weeks, I cancelled the hotel reservation because my plans changed. The certificate went back to my account with an expiration date of 8/17/19-I asked if they could extend the expiration but was told they don't do that anymore now post-merger.
Here is my data point with the travel package:
I tried to rebook over the phone and the CSR didn't seem like he knew what he was doing and said I could book any category 8 hotel, so I booked a cat 6 (post-merger category), but the certificate didn't look like it was linked to that booking. I called back a day later and they said there was a problem with the booking, cancelled it, then said they had to re-issue a new post-merger certificate that says its for the post-merger cat5. In the process, they refunded me 30k marriott points since this new cat5 certificate was less expensive than the pre-merger cat8 one I purchased. This new cat5 now has an expiration date of Feb 2020, and the CSR told me that if it gets close to the expiration, I can always call and have it extended.
Now when I tried to book with that certificate the cat5 hotel I wanted to book in Kauai wasn't available, but the cat4 one is, so I requested over the phone to book that. This time when I chatted with the CSR, he said that he couldn't book a cat4 hotel with the cat5 certificate, so he had to reissue a new cat4 certificate, and in the process also refund me 60k Marriott points due the price discrepancy.
So now, I am +90k Marriott points in my account, and have a cat4 hotel booked.