Ouch - so Open GI want it in DBE but wont tell you how to get it!? I presume you are paying them for the Transfer service
Not sure I can help easily - it depends on lots of things - why you are doing this, what are you trying to achieve, what happens to the data you can't / don't bring across, what about accounts, risk, diary, operator trails, prospects, histories etc etc etc etc. Is it policies live after 2016 but taken out in 2015 - or just NB from 2016 etc?. Sooooooo many unknowns to give you a proper answer.
You really need to be guided by the business need and their scope / requirements and, then ask OGI to deliver the service / solution to you.
Looking back - the DBE "how to" is probably the easiest part in this. The "why" and the "what" are the more difficult questions to be addressed imho - but maybe that's just me being me
I / we here can help with the DBE parameters as and when / if needed (if OGI can't / won't)