Can you switch the co-first author position after the article is published?
Journals generally don't want to change things after publication - especially things like authorship order that would cause havoc to past citations. It thus seems unlikely that your coauthor will even try, and even less likely that the journal would acquiesce to such a request. I think most (well-run) journals would also try to contact all involved authors if there is such an unusual request, so you're likely to be informed if your coauthor were to try something. Finally, even if such a change were to somehow go through, usually people take the idea and statement of co-first authors ("these authors contributed equally to the work presented in this paper") at face value, so it'd be unlikely to affect either of your careers to any significant extent (with certain exceptions). Though of course, there's the usual caveat that conventions in your field about co-first authorships might be different.