you can and have always been able to buy the base pack early to lock in the early adoptor stuff then upgrade later without loosing those awards.
I know that was in the FAQ for earlier multi Mech packs (many had tiered early adoptor rewards e.g. an extra varient and went as far as to say if you upgrade after the deadline you only get the rewards for what was purchased before), but as you say it is not in the FAQs now so it should be added in there