The 'different voltages' you are thinking of are for when the power is from different systems. For example, when the shop has both a 480/277 and 120/240 service.
The customer needs to realize that his lack of foresight yesterday does not mean you get to save his tail by tossing out the book today. Thee's no way to avoid disconnecting the old feed, and digging for the new. While you're at it, don't forget the outside disconnect.
Perhaps this time he'll let you bury some big pipe(s), rather than just slipping a little romex under the sod
(OK, I assume much - but we've all seen this before).
I say "pipe(s)" because this is a very good time to run a second pipe -preferably 1"- for future phone / computer / CATV / alarm service. As long as the ditch is dug - what's the cost of a little PVC?
I also hope the building has a proper ground rod (or GE). He already needs one - the panel makes the wire he has a feeder, not a branch circuit.