I'm assuming when you say "switch to the visa signature card from the regular marriott card" you are wanting to switch from the base Marriott Signature Visa to the Premier Marriott Visa. If that is a correct assumption, then yes, you can upgrade/change your card to the Premier card if you so choose. Based on previous posts on this issue, there were some issues early on with those who chose that route (i.e. not getting all the benefits, etc.) But hopefully, they have ironed out all those details by now.

In fact, my wife had a regular Marriott Signature Visa that I cancelled last week as she isn't using it. The person I spoke with mentioned that she could upgrade the account rather than close it.