Can you use a 120v to 240v step up transformer for a hot tub?

no you must run a dedicated line

NO

Yes, with a caveat. The current ratings must be the same, the the 110V outlet must be a dedicated circuit, i.e. nothing else on that breaker. You can safely upgrade a dedicated 110V 15A circuit to a 220V 15A circuit by re-using the same wiring. You will have to just replace the breaker and the outlet. You cannot, however, increase the current load or have a 110/220V (4-prong) outlet. Note that if you move a non-dedicated circuit up to 220 you will start a fire.

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