That exact same thing happened to me on one res change. Crazy and scary and after I'd seen preflowers too. Since then I do as quick a change as I can and no problems since.

I pump the res dry, let the tub outflow empty the grow tub via gravity, and pump dry again ("dry" meaning all that the pump can get, the bottom of res is still wet). Then I quickly dump in a few gallons ASAP. Then nute it and top it off and adjust PH a bit later.

I think it'll be fine as long as you go quick. Some people told me they keep a second container of water handy and set the roots hanging in it while they change the res. I cant lift mine off anymore, so I just sprint.