When they become available, you could order a US 4G mainboard from Dragonbox and simply swap between it and your EU 4G mainboard as needed (open Pyra, disconnect LCD cable and CPU board, remove EU mainboard, replace with US mainboard, reconnect LCD cable and CPU board, close Pyra, magic bananas). There might be some config mumbo-jumbo down the line, but that's the theoretical method due to the Pyra's modular nature.
No, you won't have to desolder anything. Simple.
AFAIK, the only overlapping bands are all 2G, so you'll only be able to call/text. No data.