IMHO and in my experience , te OEM tweeter is fragile. I have blown many. Once you replace one , you can never get the tweeter torqued down or seemingly to fit as well as the original did in its rebate and the horn of the tweeter resonates at some freqs. This happened on my 5000's and 5500's.
As to the tweeter swap , well its totally reversible so worth a try. My DSPs cost 14k or so , I bought a full set of "alternative" drivers , 2 mag bass drivers , 2 paper bass drivers , and a midrange , all the Excel line for a small percent of what i paid for the speakers. furthermore I have custom side cheecks for them.
The real nice thing is you can play and go back to ground zero after.
The metal domed tweeters are more "airy" then the Milleniums and you might find the milleniums too warm , you can use the inbuilt controls in the speaker to adjust treble balance , there is also an internal tweeter level adjustment you can fiddle with if you want more zing.
In terms of tweeter fall off in my room , I measured using the alan Parsons sound check CD and a calirated DB meter (noit a ratshack)
I used this quite often so measured a few OEM tweeters as I also use a Z-Sys RDP-1 digital parametric for various putrposes and often measured freq response at listening position.
Swapping out the midrange in my 5500's was a disaster. Swapping out the bass drivers wasnt and funnily enough a combo of one paper and one magnesioum bass driver seemes the best in terms of slam and taughtness. The bass is significantly improved over OEM , probably less accurate perhaps?
In my room , I have always found the 5500's bass to be overwhelming and somewhat flabier than either the 5000's or 6000's and the driver swap cures this.

Im pretty happy with the results so far and the sound pleases me and I thought the driver upgrades worthwhile for their cost and ease of doing.
As I say , I can always go back to the original config if I wish.

Roon/Tidal, SBT, Trinnov ST2 , Twin Devialet D premiers, Vivid Audio Giya G1 Spirits, dedicated and treated room.
Meridian gear: 568.2mm and DSP5000's acting as TV speakers.