You probably want to read ** PLEASE READ BEFORE POSTING YOUR MARKETO PRODUCT QUESTIONS HERE **
It depends on the modification you made. If the modification is outside the modules, it will change all the existing emails. If you added a new module or modified a module, nothing will happen to your existing emails and you will have to add the new or updated module to an existing email to see the changes.
In both cases, though, you will have a draft for each that you will need to reapprove)