@Ronnie Woolbright Hello Ronnie - you have a few things here I feel would be worth diving into and for the benefit of the others.
1) Changing the terms of the lease. As mentioned above, simple deal....once the original lease is up you are required to give a tenant a 30 day notice of any changes. It's already on the standard TAR form. You can do this in writing through the TAR lease extension form and then change the terms of the late fees. But, maybe do a new lease all together and put it in writing a bit more clear with a new form. I would recommend that first.
2) Late Fees. In my experience, most landlords do not charge a sufficient enough late fee to discourage bad behavior. Which do you want - the rent paid on time or late fees? We want the rent paid on time, so our late fees are very high. Since we have standardized them, we are in compliance with Fair Housing and Texas Law.
Understand we simply want the tenant to pay on time. If you want late fees, keep them low and the tenant will put no real concern in them and continually pay late making excuse after excuse. Would you figure out a way to pay your mortgage on time if the late fee was $500? Or even $5,000? Sounds excessive....and it is....but without real consequences, how important is something?
3) Zero Tolerance. In reading some of the above posts, it would appear you have become a little laxed in enforcing the lease agreement. By doing this, YOU are violating Fair Housing Laws. By bending or giving on any of your written agreements and policies, you can be accused of offering un-fair treatment. Remember, favoritism is discrimination. When you are deviating from the set agreements in place, you can be accused of offering favortism because someone is.....white/black/green/orange/married/gay/etc...
in going forward, my advice is to get your tenants on a higher late fee schedule to make it very painful to pay the rent late. Establish a rock-solid due date for rent, and let them know you will have ZERO exceptions from here on as you are forced to comply with the Fair Housing laws and can not deviate from the lease as written under any circumstances.
Good luck out there!