Exchange maigration mail users

First, let me remind you that you cannot install Ex2016 in an environment which still contains Ex2007 servers, so that sets your upgrade path through Ex2013. I would not start with decomissioning Ex2007, as that in essence means removing all on-prem configuration items related to Exchange, which you then need to re-configure after deploying Ex2016 (e.g. accepted domains). Just follow the regular upgrade procedure for Ex2007->Ex2013, then Ex2013 to Ex2016. Since everybody lives in Office 365 already (assuming you have autodiscover pointing to Office365 and you have no mail relay functionality on-prem, etc), you have no worries regarding namespace, public endpoints etc.