Hi nygobjxiii,
Currently, the Test Center version of the GMAT and the At-home option use separate scheduling systems - meaning that you could technically take one of each on the same day (although I would NOT recommend that you do that). Thus, if you wanted to switch Exam 'formats', then your only option would be to reach out to Customer Service to see if they will do it for you. While that is probably not too common of a request, you wouldn't be the first person to ask - so there might already be a policy in place to handle these types of situations.
GMAT assassins aren't born, they're made,
Rich
Contact Rich at: [email protected]