Inherited Tenants, Want to Sign Them to New Lease

[David Johnson]

I purchased a 5 BD/2 BA property in Baltimore last August and inherited two tenants who have been living in the property as a rooming house. They each pay $350 and $380, respectively, and are left over from the previous owner. I don't have a lease with them (they've paid on time every month) and, as far as I'm aware, there isn't a written lease leftover from the previous owner (doesn't mean it doesn't exist, though). 

I'm about to have the lead certificate renewed and the rental certification completed after having done a number of improvements to the property and my question is this: is it possible for me to raise the rent (I'd be looking to raise it to something around $450)? Is there a risk of any legal issues because of the lack of lease or lack of rental certification?