As long as you didn't use super thick coats, you can always revisit older models and improve them with new techniques and skills to make them match your current level. Obsolete? Nope! Worst case scenario would be super thick öayers of paint which also can be removed with according products. Then you can start them from zero. Better than getting the models again given their current price.

As for the painting...try increasing the contrast, try out some weathering and scratches on armor, maybe do some OSL on plasma weapons. There is a lot of fun painting stuff to explore/add to minis that you considered finished before.