In the Stamped Concrete industry today it has become necessary to remove old sealers prior to resealing particularly if you do not know what was the original sealer. I have tried to strip with chemicals of all types and some are not good for the environment and very messy and can ruin adjacent plants and gardens. Strippers are do not get all the material off and if the slightest amount is left behind you may encounter the same problem after applying new sealer do to incompatibility of sealers.
I am happy to say that I have found an environmentally friendly solution to this problem. The solution id wet Sand, pulverized glass or coal slag blasting with water.
My preference although a bit more expensive is pulverized glass. It gets lost in the surrounding areas and the heavy slurry is easily cleaned up. I do not own a Geoblast machine but have a company I sub out this stage of the work too. He does a great job and I feel is fair with his pricing.
Years of trying other solutions has failed. Look to my article on this site on how to reseal. There is a proper method to this also.