Using a blow-off would not be problem. I don't even think you need to switch the water, though I would anyway. For some additional protection, add some sanitizer to that water. Keep in mind that a blow-off tube is really just a big airlock.
Brody's Brew House
Primary: Pumpkin Saison; American Brown Ale(
Seconary: Don't use one, generally
Kegged: Pumpkin Porter
Planned: Either a Porter or a Scottish Strong Ale
"For a quart of ale is a dish for a king." - Shakespeare