Renters generally understand that rent can be changed when the lease runs out and should be braced for the possibility that there will be a moderate increase. If the renter doesn't understand that, they probably haven't rented much before, and they'll learn it the first time their rent increases. If you are lucky enough to go multiple years without any adjustment, you should expect that the landlord will want to correct the differential at some point and budget accordingly.

Note this works both ways. If the market goes down, if no one wants to live there anymore, tenants have a lot of room to negotiate. Vacancies are expensive and the hassle of getting a new tenant is not something many people want to handle even in good conditions.