Marti - Here is where I think you stand from a legal (ish) point of view; however I have to qualify this in advance by saying that I'm not a lawyer.

When you booked the villa originally with the previous owners you signed the Ts&Cs and sent them a payment, albeit a deposit. That created the contract between you and them for the villa rental; and unless there was any provision in those Ts&Cs to change the amount or indeed change anything else, then it isn't supported by that contract.

But then the villa was sold and so this is where it gets a bit complicated.

Your contract was with the previous owners. The new owners only need to honour that contract if the contract of sale of the villa had in it some provision to say that they would. I think this is unlikely.

This leaves you, and in fact the previous owners, on a bit of a thorney wicket as they still have a contract with you to provide you with the accomodation that you booked ... unless of course the Ts&Cs that you signed have a clause that gives them an out in the case of selling the villa. Note too that this would not be covered by force majeure.

In theory you have a number of choices:
1. Require the previous owners to provide you with the accomodation that you have booked at the agreed price; or at least provide you with an alternative of equivalent standard and quality
2. Require the previous owners to give you a full refund if they are not able to complete the contract
3. Agree to take out a new contract of rental with the new owners

Option (3) looks to be the one you are doing however I assume that nothing formal has been done so far; i.e. you have not yet signed anything given to you by the new owners or their MC. I know that sounds a bit formal and a bit of admin overkill but it probably is something that you should make sure happens. At the moment you have a contract with owner A however owner B will be providing you the villa via their MC and owner B doesn't seem to care much about how well you are looked after. And all that said there may be another twist in that the new owners and/or their MC may be deemed to have implicitely accepted your rental contract at the point they took the balance payment off you.

Confused? Yeah - it's great isn't it when you start digging down into what is actually happening.

So going back to option 3. If you get to the point of getting a booking confirmation and included Ts&Cs from the new owners and/or their MC this *should* confirm the dates, the amount and also any additional amounts payable ... for example if the exchange rate changes.

At that point it is up to you as to whether you choose to accept that new rental contract (which could in theory have a different price and a whole different set of terms and conditions) or whether you choose not to accept that new rental contract. I believe that you are entirely within your right to choose not to, and if you do you would be elligable for a full refund. However even that now gets a bit complicated as you have made a payment to the new owner/MC and so they might take the position that by making the payment you have implicitely accepted a new rental contract from them; however without having sight of their Ts&Cs I'm not sure that would stick.

If it were me I would ask the new MC to create a new contract of rental for you, pointing out that your original one was with the original owner. When that contract arrives it's up to you to decide how to take it forwards. If you are happy with the amount and the conditions, sign it and have a great holiday. If you aren't happy with it, tell the MC that you instead would prefer a full refund and itemise the exact monies you have paid that need refunding (including currency details if you have paid in more than one currency).

At least this way you confirm everything one way or the other so you know where you are up to and don't put your holiday at risk.