I think she might become an independent but still caucus with reps (like Sanders does with the Dems), so not much change.
All anti-Trump members of Congress should just switch to being Democratic, or at least Democratic-caucusing Independents.
There is no future for them in the Republican Party. As Democrats they can form a new Centrist Caucus with some of the existing Democrats and hold a bit of leverage when they make the different in who is in control.
Would she keep her seniority if she defects to democrats?
I'm sure Schumer would allow it if that's what it takes to make the flip happen. Same for Collins, Romney, and Cassidy.