Company getting acquired: would they keep salaries the same? Or re-evaluate everyone?

The startup I'm working for is likely to get acquired shortly. We get paid above market rates and it has been one of the reasons for low attrition here.

Would an acquirer lower everyone's salary to match their own hiring scale? Or are companies less likely to do that in order to keep the original staff of an acquisition happy and on-board?

It depends on the acquirer, but in most cases salaries are re-negotiated according to their existing scales.

One thing to keep in mind in case they decide to reduce your salary (and you decide to leave because of that)...

Read your employment contract -- does it have a "good reason" provision? A "good reason" clause treats a reduction in pay (especially following an acquisition) as an involuntary termination of employment, thus triggering severance.

Smart acquirers will cherry pick the best employees and keep their salaries the same (if they're already above their own scale). And smart startup founders would give you a bonus for the exit.