Switching feet is remarkable easy and it never ceases to amaze me with just how badly you can position your feet and get away with it, if you're determined enough. Of course, wider and flatter boards make it easier...
Do it whenever it feels right.
For heel to toe gybes, I started out like this:
riding natural - just after the turn
riding goofy - in the middle of the turn (!)
Now both sides I can change stance whenever, but goofy turns still feel a little alien to me.
Switching feet before the turn to make it a toe to heel carve - I can only do it riding goofy. My toe side riding is pretty weak, so I find the best time do it is when you're bearing off a little, then dig in the now toe side edge and load up again...