Adding a new tube, a 6BK4 in place of a 6BK4 B created more brightness in my ctc 17x ad, and some bluish glows in old tubes and some popping/clicking sounds (it hasn't made any sounds up to this point). It is a HV regulator, so is the set getting more volts? As in perhaps closer to the 24kv it needs? And older tubes glow more?

Are these interchangable?

Not interchangable, need 6EL4 or similar

You really need an HV meter if you are going to work on the HV regulator of a color set. HV is supposed to be set based on HV voltage measurement or in some cases off the HV reg cathode current.

If the HV is say 3-4KV above spec now, the flyback could start to arc internally, form an internal carbon track and fail.

Check