Diamond vs Aluminum tweeters for B&W 800 series

If they're two different tweeters, you can't just "swap them out" without changing the crossover to account for that.

ok, i agree.

although i found some that are described to be used instead of the diamond.

"These are factory fresh brand new aluminum tweeters made by Bowers & Wilkins and are an exact fit to the original tweeters."

They probably are an exact fit, but the crossover won't be calibrated specifically for them 

My experience with speaker design has shown me that tweeter material isn’t nearly as important as the designers choice of crossover values. A bright commercial speaker isn’t bright because of the tweeter material, it’s bright because they wanted it that way for a Wow-factor! They could easily pad down the high frequency response without changing the tweeter material. BTW, @roxy54 is correct! You can’t simply swap in and out drivers without reworking the crossover. It doesn’t work like that…..

I had 1 tweeter shattered.

If I buy only 1 new, would the sound be very different from the old one (other speaker)?

Tks.