Client has a Vertical SBX IP configured for digital phones/trunks and would like to go full IP (SIP trunks & remote phones for a second office). Is it worth "upgrading" this system? I'm supposed to propose them a new VoIP system from another manufacturer, but being lazy, would rather convert their current system.

It will do it with the addition of an 8 port ip card. I take it you are looking to go just ip phones in the second office and share sip trunks? If the second office wants it own trunks then you will should network two systems and then use digital phones not ip phones.

Yes, will keep existing digital phones @ main office and IP phones @ new (remote) office, while sharing SIP trunks from the PBX.

Shouldn't be a problem as long as you have enough ip ports...the system has to have dedicated ports for the sip trunks and the balance of ports can be shared by the ip phones, one for each phone with an active call.