lowrider57; moving hot leads from one phase to another in the panel takes no time at all for an electrician.
Adding a new box takes a little more work.
But what you described is really not complicated.
You could actually do this yourself if you knew how.
On my house it is simple. I would go outside to my meter panel. There is a large circuit breaker for my house. Yes it is that old.
I will trip off that circuit breaker, de-energizing the house feed from the meter. (I actually don't have to do this, but it is safer), Then I would go into the house to the breaker panel in my house. I would locate the breaker that feeds the line described, then remove the breaker, take the lead from that breaker feed and move it to the other hot phase in the panel. reconnect the circuit breaker, turn on the main breaker and you are done.
If you are doing this yourself, just take a volt meter with you and measure that the voltage is turned off before proceeding.
There are two hot leads coming into your home. An A and B phase and a neutral. A is on one side of the panel and B is on the other. They both measure 120VAC with respect to the neutral. They are out of phase with each other. If you open your breaker panel, you will actually see the two phase wires connected to the breaker bus bars on either side. You will see the neutral also and a ground (which is your house ground).
If you are not comfortable doing this, an electrician shouldn't charge more that an hour's labor (that is very conservative) to do this.
enjoy