I would place the frogs into a critter carrier with fresh, clean water (or old tank water) and move everything to the new tank. Do you have a filter? If so move that for sure. Once it's moved let it sit for an hour or so then add back in the frogs. The beneficial bacteria from your frogs should take over and all will be well.

I would avoid gravel entirely. Perhaps try smooth silica sand? It's much easier to clean, the only easier to clean method is glass-bottom, but I like sand personally. Looks more natural.