Changing out 208v high bays to 120v LED high bays. I want to know if I can just jump off one leg of the 208v for my 120v fixture.

The entire circuit needs to be converted to 120 volts, assuimg you have a 120/208 Volt WYE system.

One leg of the 208 volt circuit can be used, and the other needs to be taken off the circuit breaker and connected to the neutral bar.

The 2 pole breaker for the 208 volt circuit then needs to be replaced with a 1 pole circuit breaker rated for the circuit wire size.

Of course, the circuit amps needs to be rated properly to handle the new load, which being LED should be minimal.