Does rumble scrapper +2 might reevaluate after another might get switcheeroo?

Yes — Rumble's bonus keeps applying after the swap, but Switcheroo's modifier does not change.

Switcheroo snapshots both units' current Might (including Rumble, Scrapper's passive +1 to Mechs) at resolution, then applies a fixed +X/−X modifier for the rest of the turn (Rule 433.1.b). Rumble's bonus is a continuous passive — it doesn't "re-evaluate" the swap; it just keeps applying on top of whatever total the unit now has.

Small correction: Rumble, Scrapper grants +1 Might (not +2). The extra +1 only appears if the Mech is a defender with Shield from the Mechanized Menace Legend — and that also just keeps applying as normal after the swap. So if a 4-Might Mech with Rumble's buff (5 current) gets swapped with a 3-Might unit, the Mech ends at 4 + 1 − 2 = 3, and the other unit at 3 + 2 = 5.