Lot’s of good advice above! Personally, I would let them stay month to month and not say a thing since everything in the current lease remains and they sounds like good tenants. If you insist on a new 1 year lease then do not increase the rent too. I love the idea of my tenants staying in the property and paying my mortgage and in return I do not raise the rent on them and keep everything in good standing. I also like the aspect that if rents are going up around them they are even more likely to stay where they are and continue the win-win relationship. You can always update the property a bit when/if they move out and jack the rent a little at the same time if you choose to.
