They really haven't gained any value yet as they are not that old. However, I would not paint or alter them in any way. Leave them as is. They can be cleaned with warm soapy water, toothbrush if needed. Change the shade if you like. In 20-30 years they will start to go up in value. The trend(s) is telling us that the more original the piece the more value is is apt to gain.
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best............Tim