I have a solved white face but I want to swap two opposite corner pieces of the white face to form a T-shape on the back face. How can I achieve this without messing up the other pieces on those two faces?
As far as I can tell from the question so far it would appear that you mean you have solved the white face (
U) and have all of the first layer - the ring around that face is all of the correct colour - except for to opposite corner cubies like so, or a mirror image (or colour change):
...and you want to swap these two without changing the ring except at those cubies (thus fully solving the top layer).
This is quite easily remedied by just using the bottom layer, one quick way would be to perform:
R2 F' R2 D2 R2 F R2 - Singmaster notation
The first three moves will set the two corners onto opposite locations in the bottom layer with everything else out of the way; the
D2 swaps them; and the last three put everything back again.