Can you switch the co-first author position after the article is published? Can you switch the position of the first co-first author with the second co-first author of an article after it is published? 

Full story: During my PhD, I published two articles, both as the first of the two co-first author. In both articles, the second co-first author is my colleague. My colleague is highly career-driven and ambitious. Although he did not do much work to contribute to either paper (I did 90% of the experiments and analysis and writing, since these are my main projects for my PhD), but he has a good relationship with our supervisor, and managed to convince our supervisor that he did a lot. Our supervisor then spoke to me and insisted that I included him as the second co-first author. I am ok with this, since my name still appears first. 

Now, I have finished my PhD and is going to leave this research group soon. I am worried that my colleague, being career-driven and has a good relationship with our supervisor, may try some means to switch his name on these articles with me, and becomes from the second co-first author to the first co-first author.
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.