As I’ve noted elsewhere I’ve never had the ability to edit my vehicle at all, even on day 1 after I placed the order. At the time, the website still said that you’d have a few days to modify it before the order was sent to the factory. I never got it though.
I just asked my SA about it last night, and he guessed it was to discourage latecomers like me (ordered 11/1) from constantly tweaking the design and thus delaying production. I guess that makes sense, but given how uninformed a lot of SAs seem if this forum’s testimonies are true, that might be his own guess rather than an official statement.