Draft homolgation is supposed to be finalized for existing DPIs by the end of the season, per IMSA's Geoff Carter:
http://sportscar365.com/imsa/iwsc/im...-coming-weeks/
HPD will be allowed to run under draft homologation like everyone else this year. Article also shows how much BOP goes into DPI. You can make whatever changes you want it seems, but you have to re-homologate those bits in the wind tunnel or the dyno at NASCAR's R&D center.
Also, any big changes would have to be addressed with the ACO and the WEC Endurance Committee before IMSA signs off on them.