Scott, I disagree with your first statement. An existing 240/120v 1ph x-former has a grounded, center-tapped 240v secondary, while a 208/120v 3ph x-former has a 120v secondary with one end grounded.
If you mean that the POCO can open the can and reconfigure the secondary, paralleling the two 120v windings, then yes, they can re-use the x-former, along with two more.
However, with the exception of multi-family units, I've never seen a residence supplied with 208/120. Not all residential appliances are 240-or-208v rated.
It's more likely that the POCO would replace the existing x-former with a larger 240/120v one, for the mixed loads, and add a second one to provide an open delta.
On a job I'm doing right now, the POCO said any load above 400a would require a new, 208/120v Y service, at the customer's expense, so we made sure we could use the existing open-delta service.
This is an older service, feeding a restaurant, and the only 3-phase loads are a pair of HVAC RTU's, 40a each. We'll have a 100a 3ph panel for the RTU's, and a 225a 1ph panel for everything else.
[This message has been edited by Larry Fine (edited 03-16-2006).]
