Good news and bad news. I just "found" my Kentucky Rifle 45 caliber muzzleloader that has been missing for over 15 years. That's the good news. The bad news is that after sitting all that time I'm worried that getting the nipple out could be trouble. Any advice on how to get it out without stripping the threads would be greatly appreciated. Thank you.

Remove the barrel from the stock and put the breech end in a can with some good penetrating oil (Kroil, PB Blaster, etc.) and let it soak for a couple days. If you break the nipple loose and it's not turning out easily, drop it back in for another day or two.

Soaking in 50% Auto Transmission fluid and 50% acetone is about the best penetrating oil you can get.
Don't get in a hurry let it soak,.

When soaking, I would set it up so the barrel is vertical and then also pour a few ounces down the barrel to let it soak in from that side of the nipple also.

I've had great results with dunking the barrel in boiling water. The heat will do the job.

Of course, oil it up well once removed.