Apologies if this has already been asked and answered before, but can foci be upgraded, or do you have to buy a new one each time at the desired force?

i think you don'T have to BUY a new one, but if i remember correctly, you have to rebind the focus for the new karma cost.

It would have to go through the Enchanting process all over again. But presumably you would still have some of the bonuses for Enchanting that it had to begin with, eg. virgin telesma. If you made it a stacked focus, that is with a different Enchantment added to the original, it would have both functions still, but to rebind it you would need to pay in Karma for the sum of the separate bindings, including the original.