When they cancel your OC1-5 to NC1-4, the certificate automatically resets its clock to one year from the date. So basically you have one extra year to use it.
Yes, you can cancel it and get 45K back to your account, but I think keep it as a N-C for 7 nights is much more valuable compared to 45K.
Yes, you can gift it to a family member, even friends.

Is the "convert" option dead? At least, it is to me. I called UK number at least 4 different times in the last couple days and they were not able to convert OC5 to NC5 with 105K points. Every single time, I was told that it can't be done anymore.

If you have the points try making the booking with points - be sure to read the terms when booking to ensure they are refundable
Then calling and asking to attach your travel cert instead of points.

Try another country perhaps.

You think people have not thought about that? There are obvious reasons. You have not read back enough the history. People are not dumb if what you think is feasible, why no one attempts it. Common sense would tell you that there must be reasons.

It is already difficult to get an upgrade at the time OC convert to NC. It is exponential more difficult to almost impossible when it is an NC and one tries to upgrade, because the new Travel Package T&Cs specifically FORBID upgrade, among other things. In fact it does not even allow change of reservation or cancel the cert - i.e. there presumably no residual value when cancelling an NC according to the new Travel Package T&Cs.

Anybody has an OC and does not have any immediate need of a booking, should wait as long as possible before doing anything to the OC, whether to upgrade to higher cat or to just convert it - until you need to book something or the OC is expiring - whichever comes first.

I called the UK number yesterday and converted my OC6 certificate

I called the UK number yesterday and converted my OC6 certificate

I have converted an old Travel Package into a new certificate (NC6) - are there any data points of partial refunds being available? - eg if we end up staying at a NC4 property?

Hoping to use it at the higher level, but (most likely) looking for a European property that is suitable for 2 adults + 2 children... which generally seem to be Cat 4/5

I see...so I missed the fact that travel package certificate upgrades are not allowed at all in the new world...well that blows big time.

Officially it is not allowed but a few have managed to get it done. It is exponentially more difficult once it is an NC so unless you have no other option do a conversion from OC to NC. Straight OC to NC conversions of course are explicitly allowed and I do not believe there is any system restriction preventing up conversions while going OC to NC so it is just a matter of finding an agent willing to process it. I suspect once you have an NC there are system restrictions which require a supervisor to manually process it which likely will not happen without an excellent reason, probably involving Marriott having made some mistake for which an argument could be made for them to make it right.

A data point for everyone. I had an OC9 booked last August and attached to a reservation that I never intended to use. Yesterday I called the US line (not UK) and asked for the agent to convert my OC9 to a NC5 (QP91 code) - this is because I have a Cat 5 hotel reservation that I will actually use for a week and wanted to get the 60K points back as well for going down to a Cat 5. I know this is the reverse of what most people are trying to do in getting a higher certificate. The agent on the phone said she would order me the QP91 - Cat 5 but ultimately ordered me a NC6 (QP99) which is technically the right conversion in their system. I got disconnected during that call unfortunately and it was processed. Note that prior I asked for the OC9 to be detached from the existing reservation - it was tricky as the agent kept wanting to cancel the reservation. You have to really repeat "detach!" and "don't cancel the existing reservation" as I understand from the thread that this will create a big mess.

I tried yesterday and today over several calls/hang ups for an agent willing to convert my NC6 to an NC5 and refund me the 60K points. Some are the agents are legacy SPG and have no idea about this and once they ask for help come back and tell you it's not possible. Finally found someone who got the OK from their supervisor to do so - it was incredibly complicated. They had trouble deleting the NC6 and then finally were able to do so and order me the QP91 - took 45m. In the end it all worked out. I got the 60K points back from downgrading (or rather "converting" as I put it). I also learned in the process that if an agent agrees to do it for you - ask them to open a case number and document it so it's on the record in case you get disconnected or they can't technically do it and you have to call back.

To a NC6? For additional points?

Not to ask repetitive questions, but it helps to see additional specific data points. And hopefully stops the same questions from being asked over and over again....

Upon conversion to new TP certificates, not only are upgrades/downgrades infinitely more difficult, but also cancellations to get back the residue/surrender points (45K for NC4, etc.) are infinitely more difficult (or even impossible).

I might have missed some data points here but how do you know that getting back the surrender points is infinitely more difficult?

Very good quiz question. ;)