i've never dyed over a piece that was previously sealed and top coated.
most use an oil based dye when possible, i use some alcohol and water based gel antiques as needed for my projects. it can be dipped or applied with a sponge or wool dauber. dyed leather when dried is sealed and top coated. the sealant and topcoat you use will depend on the sheen and luster you desire and what dye you used, there are many.
once your holster goes black, it'll never go back.
Depends on the results you want. i got satisfactory results from a novice neighbor who died an El Paso holster for me. It's not perfect, but it is darn good enough for the reason i needed the holster to be black instead of tan. I can ask what he did, but the gist is he scuffed up the finish on the leather before dying and dyed it. I can see some scratch marks, so there's probably some extra work that could be done to clean it up better, but overall, not bad, not bad at all.