[Don Pensack]

Though the films on these eyepieces are mostly oxides, some layers do react to sulphur in the air and other contaminants. 20 years is a little longer than they last here in LA, and they might last longer in some pristine environments.
If contemplating replacements, know that there are a variety of orientations for filters today that might influence your choices:

Narrowband filters with Wide bandwidths: These are good for smaller scopes or for observers who prefer the aesthetics of having more stars in the field of view or for use at higher powers. Not the best contrast possible, but a huge improvement over no filter. Examples: TeleVue Nebustar, Baader UHC-S, Astronomik UHC

Narrowband filters with medium bandwidths. Though the overall field of view will be darker, contrast on nebulae is improved and performance is near-optimum, though star images are suppressed more. Examples: Lumicon UHC, Thousand Oaks LP-2, Orion UltraBlock. Usable up to about 10-12X/inch.

Narrowband filters with narrow bandwidths. The maximum contrast, but the greatest suppression of stars and the darkest field. Great in the largest scopes or at very low powers. Example: DGM NPB.

For O-III filters, there is also a hierarchy of bandwidths. The widest ones are close to the width of the medium UHC filters, but don't include the 486nm H-Beta line the way the UHC filters do.
Wide: TeleVue O-III, Astronomik O-III
Medium: Lumicon O-III, Orion, O-III, Thousand Oaks LP-3, DGM O-III
Narrow: Baader O-III, Celestron O-III (these are so narrow they exclude most of the light from one of the two O-III lines and should be thought of more as wider photographic filters than visual filters, though they are usable visually)

This plethora of choices (and i only mention a few of the now 25 brands) wasn't available 25 years ago, so you have to choose wisely based on whether you want more contrast, or a brighter field with more stars. There are valid reasons for both approaches.

I sorted it out by choosing to have both for different purposes. For instance, some of these filters have a lot of transmission at the 656nm line of H-alpha. Some have little or no red transmission shorter than 700nm, so you see none. It may be nice to have some red transmission if you have a large scope or are looking at an object with profound energy of emission at that wavelength.

Lumicon is still the benchmark for highest transmission and near-perfect bandwidths, but have little or no red transmission since 2005. Lumicon is now back in business with new owners, Marc & Debbie Neveux and they are shipping filters again as are their retailers.

Don Pensack