Starting point - an old non working PSU

Base plate removed

New Exxos supply, placed on base to mark out hole for regulator

Hole marked

Brace the plate, so it doesn't bend when you drill the hole

If you have a proper debur tool (I can't find mne) use it, otherwise an oversize drill will work just as well.

Deburred.

Thermal paste ready

Desoldering the leads needed for the new unit

Ready to be reused

Atari side resoldered

Top view

Old switch de-soldered

New switch ready

New switch soldered

New wire soldered to power socket

Thermal grease applied

Note: if the hole you drilled went through a sticker, you need to clearance it

Like so

Finished article

All working :)