Just one thing to bear in mind, the UK phoneline tends to be nowhere near as helpful as the French once so make sure you call them.
ETA - just realised you booked through a TA.... not so sure on that - only ever book with Disney.
Most people do manage to do upgrades especially if they have a little bit more to pay, but it does depend on when you are going (if it's before the 60 day limit for cancellations). Also, please be aware that booking now on an offer and hoping a better one comes along is risky - earlier this year when there was a flash sale there were a few people who tried this and Disney wouldn't budge and change their bookings
..... From what I can gather, they said that they do not have to change them by rights, if you wish to cancel (pay the 10% or whatever it is) and then rebook on another offer that is fine.... So, I guess the rule of thumb is be happy with what you are booking, just in case you can't change.
Hopefully though, this won't be an issue but I'd hate anyone reading advice on here to book and then upgrade and think it's a matter of course as it does depend on which CM you get on the phone sometimes (which is silly I know)!
We recently managed to change our booking offer (via the French line) which meant our balance reduced by almost £300 (2 x rooms at the DLH). We were lucky though as we were just outside the 60 day period and had not yet paid the balance due to this - the CM said that because the balance had not been paid, we were within our rights to change to a cheaper offer eventhough we weren't upgrading or anything....
Good luck!