if the lease expires between now and next 3 month, you may be expecting a slower listing on the market. $45X18 = $810 loss from you to keep them for another 18 month.
2 actions I would do if I were in your position.
1) Check the rental history of the area in the past 1 year period to understand the rent difference between summer and winter and days on market for a full evaluation of this Market you are about to face.
2) What would be a reasonable rent in the market today? will this $45 deduction hurt you much in cash flow?
3) I would take this opportunity to REdesign the lease and expire the lease in summer time when rental rates are at its peak instead repeating again (12-month lease) in winter time to allow tenants play this game on you in their advance.
After reviewing these, you should have a good idea about the lines and should help you find a direction.
I would keep tenants if they are good and takes care property well. That's something you can't buy w/ $45 dollars.