Back in 2006 I had a MAN-MCO return booked in PE. I decided to make the jump to Upper Class & phoned VS to do the change. Initially they wanted to charge me a fee (think it was £50 back then) for this, but I pointed out I was not changing either of the flights / dates, merely changing the cabin I wanted to sit in.
They waived the fee and moved me from a K to a Z fare upon payment of the relevant amount. ISTR the booking in MMB changed instantly to an Upper Class one, but the new e-ticket & credit card charge took at least a week to happen.
I should probably have not made that call as I've only ever travelled in Upper Class on VS since! Waiving that £50 fee has been a good little earner for them in the long term!
By contrast, I have reward bookings again for a MAN-MCO return in June. We're returning on the VS76, but noticed that G's had become available after booking on the later VS74. Called VS to try and change and it was set in stone £35 per person to change flights, even on the same day. We've stuck with the VS76, as an extra hour in the theme parks is not worth £70 - that will buy a meal / a few drinks.