Andrea, I had a Twitter conversation with Lauren yesterday. LEED geek talk! I went through a similar scenario for a v2.2 project. The building was already CS certified. The tenant would take over the entire building and pursued LEED-CI certification. It was due to the fact that the building already had all the site credits built-out and claimed, and so was the core (all water fixtures were in) and common area HVAC and lighting controls. So I really think your project will be more appropriate for IDC if the site, core+shell are already there. Maybe a conversation with GBCI won't hurt?