Glasgow To Orlando - Aircraft Substitution

Hi,
In a couple of weeks I'm flying from Glasgow to Orlando on VS71.
When I log onto manage my booking, it says the flight is scheduled to be operated by a two cabin A330-300.
But when I've checked V-Flyer and the Virgin website over the last couple of weeks, I've noticed that the flight has been operated by a 747-400.
Does anyone know the reason for these substitutions?
We've got specific seats picked on the A330-300. The seating plan on the 747-400 will probably mess things up a bit.
If they do change our plane, when are we likely to find out.
Thanks,
varig_dc10.
In a couple of weeks I'm flying from Glasgow to Orlando on VS71.
When I log onto manage my booking, it says the flight is scheduled to be operated by a two cabin A330-300.
But when I've checked V-Flyer and the Virgin website over the last couple of weeks, I've noticed that the flight has been operated by a 747-400.
Does anyone know the reason for these substitutions?
We've got specific seats picked on the A330-300. The seating plan on the 747-400 will probably mess things up a bit.
If they do change our plane, when are we likely to find out.
Thanks,
varig_dc10.