Can retinal detachment be prevented if you already had a tear?

Usually, yes: A retinal tear can be "surrounded" or walled off by laser treatment which acts as a barrier to prevent the remaining retina from becoming detached. It is successful in most, but not all, cases. You regular ophthalmologist may be able to treat this, or they may decide to refer you to a vitreoretinal specialist.

Yes with a caveat: If the tear has been properly treated it will need to be watched and seen immediately for any changes, but baring new tears and the treatment of anything new that could occur, it should stay attached. Re-attachment is very successful today as well. Get to your retina specialist for any new symptoms!