Dormer attic window repair help

[Reeves2k2]

Hi guys right need a bit of help please

I have a flat roof dormer in my attic constructed in the 80's typical build studded frame and plasterboard the window is old wooden and rotten so will be replacing it
I have the replacement window upvc type
And I have only just noticed this issue
The new window is the same dimension as the original timber one but the frame of the original is wider in diameter so once the new window is in place the internal plasterboard cheeks will be nearly flush up to the glass there is about 10mm max
The original as it is at the moment has barely any frame showing on the sides

I am a plasterer by trade so removing the plasterboards aren't a problem my question is I will need to widen the cheeks slightly by maybe 12.5mm on each side to make it all look symetrical from the inside

Can this be done or is there any trade tips that will help with the issue

Again the window will fit with the existing but the internal cheeks will look odd only having a little bit of frame showing on the sides and have more on the top and bottom

Thanks[/b]