Installing a new heat pump with a contractor and he said he can convert a 120v outlet to 240v outlet on site without bringing new wires from electrical panel, heat pump specs is attached and it has a 5 amps circuit and 15 amps overcurrent protection. Contractor says as long as wire gauge is enough for the amperage, it should be fine. I chatted with other contractors but none of them have brought up this option before. Want to know if this is a safe practice?
IF the outlet is the ONLY outlet on the circuit, it's perfectly fine to change it to a 240V outlet (or disconnect, given it's for a heat pump) and change the breaker to a two-pole 240V 15A breaker - all your 120V wiring is already rated for 250V if not 600V. Since 15A/14Ga is the minimum wire size for house wiring, the wire size is already adequate by default for a 15A circuit.
If there are other outlets on the circuit, then there's going to be a problem, or more changes / rewiring will be required.
The white wire should be remarked red (preferable, IMHO, more obviously not just tape for tape's sake) or black (or any other "hot color" so not gray, white, or green) at both ends.