If you update a stack or a plugin, does that affect anything that already uses it, or would that only be applied to whatever uses it after the update?
It applies to anything that already uses it.
Update the stack then as soon as you open the project the stacks reflect that update.
This is a good thing usually but there maybe circumstances where you need to be aware of ‘radical’ changes that might need user input.
I’ve just updated my Foundation stacks and it broke dozens of pages because the update made a change to the Panel (legacy) stack which broke quite a few other stacks including some actual Foundation stacks.
OK, this is good to know. Sorry hear about your broken pages. Thank you very much!