Free air resonance?

Going by Fs alone to predict performance will not do anything for you. Unless you're absolutely sure that the other parameters do not change, a new driver can throw off performance. The biggest change can occur if the impedance curve is different, especially at the 15khz and up range. Frequently, this is where the tweaking is done - parallel resistors, Zobel circuits etc. If such corrective crossover elements exist, a tweeter with different impedances at this range can change things for the worse - or better if the planets align - regardless of a lower Fs.

Ditto on the opinions that going to a tweeter with a lower Fs is at least a wash.

And, I am of the opinion, like Gregm, that getting the crossover point further away from the resonant frequency is a boon.

At the end of the day, unless the speaker was stressing the tweeter, you are not going to realize any benefit now, but if you ever want to reengineer the crossover, you have so much more room to play with now. You are moving to a tweeter which would allow a first order crossover, though that is not the question at hand.

I agree with all of the comments made above : )

Greg and Joe are right on the money regarding avoiding driver resonance. The further away from resonance that you can get, the better off you are.

Gs is also right on the money. He points out factors that i was trying to stress in a previous post that Mitch made regarding this same subject. That is, if the crossover has specific "optimization circuitry" for the existing tweeter and / or the impedances are different at the point of crossover, you could run into problems. Sean
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There are many other factors, which need to be considered. How about Seas efficiency? If it's different from Vifa, you may change the balance of your speaker. I wouldn't change the drivers of high performance speaker. The maker definitely was matching the units or spec. ordered the units. The crossover was most likely balanced to match the particular drivers.

In the original design of the speaker, avoiding resonances is
desired for the reasons Sean, Greg, and Joe... have stated.

However, Tkwolek has a point here.

If you are going to replace drivers with drivers of a different
design - then you effectively have to re-engineer the entire
speaker system. If you are up to that task...

However, the original designer certainly took the resonance
characteristics of the original drivers into consideration
when the speaker was originally designed and made provision
for same.

Substituting drivers into a pre-existing speaker system is
probably an "iffy" proposition - unless you know an awful
lot about speaker design. It's not an "amateur's mod".

Dr. Gregory Greenman
Physicist