You can not force a tennant to sign a new lease. You can give them the option that you intend to end the month to month arrangement absent their signing the lease (after considering the length of notice required by law and lease).
Don't forget that a tennant has rights to your property under a month-to-month lease and some of those are stated in the lease, but there are some ofthers that are statutory. I would make sure you really want another lease as you may want to feel out the people for a while and then start to muscle the 1 or 2 that don't pay timely and try to leave those other tennants alone, if possible. If you want to remodel unit at a time, you have a real opportunity to determine which tennants you want to stick around. I call this hi-grading. You slowly dump your worst renter and then move in a better one (maybe after doing some work on the place).
How much do you know about the leases in place and there related terms? If these were 12 month leases that then went to month to month after the primary term, they other aspects of the lease generaly hold. Has the seller provided them?
I would only make changes to a lease if they were really meaningful to you. For example, are they paying late? If they are not, is it that big of a deal adding a late fee and sending a negative day one message to your tennant? There are some advantages of not being tied to tennants for the long term, and one of
If I were you, I would make sure you send a letter to the tennants (signed by the seller if possible) stating where each new payments should go.
If you want to remodel, you may opt to just raise the rent or evict... clearly one of those is faster.