I am so glad you got this sorted out in advance.
We also booked in January and were told in March that our flight was cancelled and had been rebooked onto the earlier flight to MAN. That was fine as it was only 3 hours earlier.
We flew home on Friday night and when I tried to check in online last Thurs it would not let me. So we turned up at the airport 3 hours early - to chaos!
There were staff going round telling all the MAN passengers that the flight had been overbooked by 107 people and did anyone want to go Gatwick an hour later and be sent by bus back to MAN. The compensation was a free return flight to anywhere in the world except Australia for use anytime in the next 18 months. It was tempting except for the fact that we were travelling in a party with a child and thought it would be unfair. Also the staff at MCO were telling people the bus trip from LGW to MAN would be 4 hours (seemed VERY optimistic to us!)
It took an hour and a half for us to get checked in and there were still people behind us waiting. I don't know whether they got enough volunteers in the end or whether they had to take some people off the flight that were unwilling.
The whole thing was complete chaos at the airport. Unfortunately Virgin are the only airline to fly direct from MAN to MCO so there isn't much of a choice!