Hi @Katie1064
Exactly as @Joan2709 has said - it shouldn't be a problem. Don't have them make a new booking, just use the Change Reservation function to extend the present one (this also ensures that they don't pay extra once-off charges, like cleaning fees).
Just one thing to add - I find that some guests don't know the app very well and battle to send a change request. If you know until what date they'd like to extend, you can send the request from your side and make them the party that just needs to click "approve".