IMHO, this is not an issue of 'grandfathering'; 480V ungrounded deltas can be compliant with current code.
The questions are:
Does it offer benefit that outweighs the risks?
If the ground detector lights indicate a problem, is there someone keeping an eye on things?
How expensive would it be to retrofit a grounded service?
IMHO if they don't need the continuity of service benefits of an ungrounded system, and if they aren't _required_ to have an ungrounded system (eg cranes over combustible materials), then I'd push for a grounded system.
Consider installing a zigzag xfrmr to establish a referrence neutral and dropping in a GFCI MAIN breaker.
Delta pretty much requires a knowledgeable player on site to jump on faults. Since this occupant is not a manufacturer there is scant chance of that.
However if the customer is using 480 solely for lighting and A/C they probably won't bite.
The danger is if their maintenance man plays with the high bays and screws up. The significance of the indicator lights has to be driven home. I'd wire in an alarm contact on each branch of the indicators: stop working on the system -- call electrician immediately.
For a good article on corner grounded systems see this data bulletin by SQ-D:
Corner-Grounded Delta (Grounded B Phase) Systems Class 2700 Publication # 2700DB0202
I'm assuming this is utility supplied. Will the utility convert their XFMR to a Grounded B-Phase??? If not, that's that. I'm guessing they would push to supply you with a wye system. They might want to get rid of their old ungrounded delta systems. Maybe offer some incentives??
Converting to corner grounded could be more trouble than it's worth. Are there any fuses in B-Phase (other than overload for motors)??? They all have to come out. Breakers need to replace them or single phase fusible equip. Your local inspector "MAY" let you slug the fuses. Higher than normal fault currents on first grounded fault. You need a main bonding jumper in the service. All B Phase conductors now need to be identified as white.
The offices and lights probably operate off a grounded systems via XFMRs so no problem. Is any equipment supplied directly from the ungrounded system?? That's what you need to look at.