I'm no expert on UA but I think this is going to be a tough ask. Though I would certainly ask.
August is a long way off and I'll bet the airline will respond by saying they fully expect their MAX fleet to be fully safety-certified and in service by then. And if it doesn't work out for the MAX, they'll probably either have replaced them or will offer the same kind of accommodation they're offering for changes now. But in June or July.
Also, though it might be futile, I would mention to the wife that once they get sorted out, it's likely the MAX will be under such scrutiny that it's likely to end up being the SAFEST plane flying.