Say that I choose Swamp for the basic land type for Magical Hack. From my own thoughts and looking around, it seems like there are 2 possibilities:
All flood counters instantly switch to using the other land type: All flooded Islands become Swamps instead. New counters also turn lands into Swamps.
Previously placed flood counters are unchanged (The respective lands remain as Islands), but newly placed ones will turn lands into Swamps. You can override the older counter by placing a new one on the respective Island.
I was wondering which of these was the case.
In addition, what happens when Quicksilver Fountain leaves the battlefield after being hacked?
Do the flooded lands remain as Swamps, or do they revert to Islands due to Quicksilver Fountain not being on the field anymore?