IME of designing, modifying and buiding speakers (often with Polk's) it will be a bit of a crapshoot.
The challenges include but are not limited to:
If the drivers have different sensitivities, which is likely, the LSI9 x-over will yield a non linear frequency responce.
If the drivers are different impedances then the LSI9 x-over points will shift as well as change the higher slope to a lower 6db slope, leaving a gap between bass and tweeter output.
IMO, you are better off installing the better tweeter and then upgrading the existing RTI8 x-over with better parts as well as changing the tweeter part of the x-over to accomadate the new tweeter.
It may be fairly easy as the two tweeters (RTI8/LSI9) may be the same impedance and sensitivity. In that case you would simply have to upgrade the parts in the existing x-over.
At the very least you would need a Digital Multimeter, a SPL meter, and a CD test disc with 1/3 octave test tones on it.