I could see this being possible on check in
Member services won't talk to anyone but the member, but the front desk will, of course, talk to the guest of record and they do have the systems access to make changes to the reservation. If they haven't been well trained, or they forget, or they are bullied into in, they can access the reservation and change it. Its very rare that we've had reports of it happening, and its a pain to get it taken care of, but the owner can work to have those points restored as their use wasn't properly authorized.
DVC has made system changes - and its possible that they closed this hole in one of the changes.
As to making changes by working with your owner, it can be difficult to find someone now who will work for you - it can simply be difficult to find the person you need with the points you want, but if you can find a private individual who will try and work with you, that's great. You should still understand the limitations of their ability to work with you - there simply may be no availability to move your reservation a few nights, they may not have the points to change your studio to a one bedroom, you may be late in their use year where cancellation means they will have only a small chance of re-renting your reservation. Willing is awesome, but probably the minority of rentals. Willing and able when needed is rarer still.
I rented out my point through David and then rented points from David to get an in demand reservation (HHI in Summer). The non-cancellation was scary, but since we didn't need to cancel, it went off fine. I'd really recommend
trip insurance - which will eat into savings, but will make it possible to cancel.
One thing sort of mentioned in this thread but not explicitly stated. People new to DVC (members and renters) are often surprised how tight availability can be. You cannot decide now that you want to stay at BCV for Food and Wine and expect to get a room for a week. It is possible to find distressed points on the market for less money (distressed meaning they need to be used quickly before they expire), but to use them, you are very likely to end up at SSR (which as the largest of the resorts, often has availability late), may have to resort hop in order to get a complete week anywhere, and may not even get that. David has this chart:
https://www.dvcrequest.com/how-soon-should-i-reserve.asp