converting outdoor 110v to 220v

If you do not need to serve any 120v loads you may be able to simply reidentify the white wire to black and swap out the breaker and receptacle.

You probably have a hot, a nuetral and a ground feeding the 110V receptacles. I'd be inclined to reuse the exisitng wiring and just put black tape over the white wire of the neutral so no one mistakes it for a neutral, and then change out the branch circuit breaker to be a 2 pole, and the 110V receptacles to 220V units.

OTOH, its possible that someone ran 220V out to the receptacles already. You just never know whats there.

Either way I would think any electrician could easily solve this problem.