20 inch ebike wheel motor exchange

Depends how the limiter works. If it knows about RPM of the motor/wheel then the increase in diameter will effectively raise the top speed.

Example - if the motor's limiter is at 18 mph on the 20" wheel, then that's 63 inches circumference.
18 mph is 19008 inches/minute.
That works out at 301 RPM on the motor.

So reverse that, 301 RPM on a 29" wheel (which is 91 inch circumference)
is 27,391 inches/minute is 25.9 mph.

However there's always a tradeoff. In this, the cost is in torque/acceleration. Your bike would take much longer to accelerate, and would "stall out" sooner as the grade increases.

Additionally, some locals have legal restrictions about the top speed a pedal-assist motor can provide. 25 mph may be faster than your local laws permit. While you're very unlikely to be pinged for the bike, if there was an accident you may be legally "at-fault" even if its not your fault in a road-rules sense.

More practically, your wiring may not be long enough for a larger frame.