"I'm fine with my contacts" - do I need another evaluation?
Yes. Contact lenses in the United States are regulated by the FDA because they are medical devices that come into direct contact with living tissue (your cornea). As such, they need to be re-evaluated prior to a release of a contact lens prescription.
Asymptomatic conditions including but not limited to corneal neovascularization, corneal hypoxia, or corneal infiltrates can develop over time from ill-fitted contact lenses and problems can be spotted early on prior to any corneal damage or vision loss. Lenses that have worked well in prior years/decades may not necessarily be suitable in current and future years.
Contact lens prescriptions are more than just numbers. For example, the base curve (BC) and the fit of the lens matters. The relationship of the contact lens with the cornea can change over time. Should the lens be too tightly fitting, poor tear exchange occurs and can make the eye more prone to disease. Should the lens have excessive movement, it may compromise vision or worse, induce irritation and the formation of giant papillary conjunctivitis (GPC).
The type of lens material, diameter of the lens, and the ocular surface such as dry eye and ocular allergies all can further complicate successful contact lens wear, and warrants further discussion.
For reference and according to the NJ Division of Consumer Affairs:
"Federal regulations stipulate a contact lens prescription is valid for one year. Once the optometrist has arrived at your final contact lens prescription, your optometrist can indicate an expiration date based upon your eye health status on the written prescription. That expiration date may be less than one year."