Depending on how the existing box was wired, you may be able to de-energize it without going all the way back to the panel. And, there is a fair chance that the line from the panel also feeds other devices.
Find the Feed Line - It may be that the line to the range hood is fed from some intermediate box in the circuit. You can try to find this out by opening the circuit breaker that controls the hood and seeing if any other lights, outlets or appliances also lose power. If so, are these closer to the panel than therange hood? If so, there is a good chance the wires to the hood are attache din the box feeding that device. You could also use a circuit tester, but that is a bit more complicated.
Find the Hood Line - If you find another junction box that feeds the hood, you may be able to disconnect the cable that is dedicated to the hood at that earlier junction box. BE SURE THAT ALL WIRES IN THE JUNCTION BOX ARE DE-ENERGIZED! You can use a non-contact tester to confirm before handling any of them.
Also be sure that there are no other devices downstream from the hood (i.e. also powered by the hood line you are disconnecting). It may take some trial and error to find which cable feeds the hood. I would take a picture of the wires, mark each with a masking tape label and carefully experiment (only work on the hots, leave the neutrals alone at this stage).
Disconnect the Hood Line - Once you have identified the cable that feeds the hood (and only the hood), you can disconnect it (now disconnect both hot and neutral). I beleive that in most jurisdictions, you can cap and tape those wires, and label them as not functional. Then you should be able to do the same with the wires in the old hood box and then seal that box up without access. [I am not certain if the Code recognizes this and welcome someone weighing in on/editing this.]
An alternative is to remove the de-energized wires from both junction boxes, but be sure to label them as obsolete so that anyone finding them later understands.
Run a New Line - Now you can run a new line to the new box, either from the old source of the hood power, from some other line that is not overloaded, or as a home run line from the panel.
Seal the Old Box - Once this is done, you should be free to seal up the old box without access.