David,
Unfortunately, it is a tad more complicated then slapping a neo in place of midrange speaker. That said, it could be done. My guess is that the 5 1/2" drivers are set up to go lower than 250Hz which means it should NOT be done. You need to start by finding the lowpass frequency and then if above 250Hz you can match the sound pressure level (SPL) of the neo to the midrange driver, and lastly match the impedance of the likely 4 or 8 ohm mid range to the 5 or 3 ohm neo (depending on variant) to minimize impact on bandpass filter.
Here are a couple ways to figure it out lowpass frequency, one by calculating the other by measuring. Figure the cutoff frequencies of the pass band for the midrange by looking at values of appropriate inductor and capacitor. There are online calculators or calculate yourself: cutoff frequency = 1 / (2 * pi * (square root of L * C)). If the high pass is above 250Hz, you should be good. You could also reverse engineer by playing a 1 volt sinewave in the midrange and then lower the sinewave frequency and when it gets to .7071 volts note the frequency as you are the cutoff of the high pass. Again if above 250Hz you are good to go. Likely the answer is not above 250Hz.
Next concern is matching sound pressure level. You could play some 1 Khz test tones and use Room EQ Wizard to take some measurements of the SPL or run sweeps with 5 1/2" drivers. The neos are quite sensitive so you will likely need to attenuate (voltage divider like an L-pad) to get a similar SPL for the same input as the 5 1/2" driver.
Oh and when you add a voltage divider to match SPL you are effecting the bandpass frequency, but if you make the divider (top and bottom of L pad) same resistance as existing midrange driver resistance and mid portion of L pad to neo + you can minimize resistance effects of new neo driver on bandpass filter.
