I have 2 older Knights. 1 is a very old one that doesn't even have a model number in .50 cal and an other one that I just acquired in .54. That one is an LK-93 Black Knight. Both of these rifles came with the #11 percussion cap ignition. When I got the .50 I bought the conversion kit that uses the 209 with the red plastic jacket. And the only reason I did was because my #11 caps kept coming off while hunting.
With the .54 I was having the same problem but through old age and experience I learned how to solve this. I simply took my file to the nipple and with just a couple strokes and rechecking with a cap I took a few thousands off the nipple and also left a slight burr on the mouth so the cap is held securely in place. Once fired the cap comes off easily. Unfired caps now take a bit of effort to remove.
I too do not see the need to switch over to 209 primer ignition unless that is what you really want.