It really depends on the restaurant and crowd.
We visited Disney during Xmas with another family this year. They decided late that they would be going to Disney. We had booked at the 180-day mark. When they booked in September or October, they initially tried for 8-person reservations or to try and get reservations close to ours. They were able to get an 8-person ressie at Teppan Edo the day after Christmas. The rest of the restaurants, they either booked restaurants in the same park or the same restaurant with a different time.
We walked into Il Mulino on Christmas Eve and it was far less packed than we expected. We told the hostess that we were together, but that we had booked our reservations separately. "Can we sit near each other?" She did one better- she sat us all together.
The other family had a reservation at Sci-Fi the Saturday after Xmas and called to see if anything had opened up at Prime Time so that they could eat with us. Nothing for four at our time. They did have a later reservation, so we missed our FP+ for Tower of Terror, but we all got to eat together. We did get seated pretty late, though, but that was because the restaurant was packed.
At Tusker, we told the hostess that we were together. Just like at Il Mulino, they sat us together.
They couldn't get into Coral Reef. They tried calling but the CM told them it was way too packed and there was nothing open - they were booked solid. This was two days before NYE.
As for BOG... well, you can guess what happened there. We ate there and they went to Crystal Palace.
Now, NYE was quite the surprise. A few days before NYE we were in EPCOT. We had success with Il Mulino and Prime Time and decided to test our luck. My family had a reservation for 4 on NYE at Rose and Crown. The other family had a reservation for another restaurant (which is slipping my memory right now.) Anyway - we went directly to R&C and spoke to the manager - Mike. We told him our situation and asked if it was in anyway possible to squeeze us in. He told the family to cancel their reservation at the other restaurant and ask for him on NYE. He would help us out. We returned on NYE - nervous and eager. We asked for him and he told us that it may take longer to be seated, but that he would make the necessary adjustments. He told the hostess to slot my family in for 8 rather than 4. We expected to wait a while. In fact, the girls ran off to the bathroom and the guys bought some beers. We were called 10 minutes later. We tried to give Mike a $20 for his troubles, but he told us to add it to our tip instead. Such an awesome guy! We all ate together in what wound up being a fantastic NYE dinner!
So, it really depends. Some restaurants are more open to others. In some, you may wait a while, but then get seated together. I feel like converting a table from 3 to 5 is much easier than 4 to 8. It should only be one table (I think.) But, thinking back to Coral Reef, it's mostly tables of 4 with a few booths. You can add a chair to the booth, but I feel like it may be a hazard (the aisle is very slim.) I'd have them call and try and change the reservation to be safe.
