VPN, no VPN, does not really matter. The question is really this. Which building is connected to your telco provider? And how much bandwidth between offices.
If only the old building is connected. Leave the old asterisk server there. The new office can ‘hang’ off the old office, phones register over the VPN connection. (mind the bandwidth consumption)
If the new and the old building are connected (and telco is not letting you move the DIDs) put in a new asterisk server in the new building. Connect the old and the new via IAX, home phones in the old office to the old asterisk, new office to the new asterisk, and create a dial plan so “internal” calls go across the VPN, and “extenal” go out the appropriate local connection to the telco.