Just change them and see if they sound good to your ears or not, 4000 Hz is still a fairly high crossover point, I'd give it a try anyway and who cares if it might cause a wrinkle in frequency response as long as you like the way they sound with the different tweeters. I doubt you'd hurt anything unless you're always playing your system at full pin all the time. Oh and on the crossover if there is only a single cap on each set of tweeters then just change the cap with the tweeter, then it will still be at 6000 Hz