Howdy, Jax!
In answer to your query of whether you can melt out lubed bullets and re-do with pan lube for blackpowder loads, it would all depend on the SIZE and DEPTH of the existing lube grooves.
Blackpowder loads apparently need larger amounts of lube to function properly and to minimize bore fouling, especially in the longer barrel lengths. If your bullets have relatively large, deep, and square lube grooves, I'd say YES, you can do it. Many, if not most bullets designed for smokeless loads have very shallow grooves, however, and will not work well for BP loads because of the minimum lube.
As for what lube works well, there are lots to choose from. Some guys like Bore Butter or Bore Butter/beeswax mixes, others like the home-made lubes using beeswax and olive oil, etc. Lots to experiment from. Some say to stay completely away from ALOX type lubricants because they lead to severe bore fouling with BP.
Some shooters favor a grease "cookie" wad under the bullet base. a thin card wad is used to separate the powder from the grease cookie.
HTH
John