+1 to AR. Market rent is what matters.
Assuming it has gone up, I'd give them a discount (not having a vacancy and delaying turnover costs will save you quite a bit), but still increase it to just below market rent. Print something like Zillow's rent estimate, rentometer's estimate, etc. and bring it to them with the new lease addendum (which merely says two lines, rent to be XYZ as of DATE1, lease to be extended to DATE2, all other terms and conditions to remain the same, or something close to that effect) and explain your costs have gone up due to higher property taxes, rent in the area is up, show them comparables, let them know you are keeping theirs below market rent, and have them sign addendum.
It's pretty straightforward. :)
And, of course, if rent hasn't gone up, you can just have them sign an addendum that just changes the extends the lease length.