We usually find that customers who alter crossover components are not fully satisfied with the results. They find that some aspects are improved, but others made worse. A classic case of this is when a polypropylene or other very low-loss type substitutes an electrolytic capacitor. We all know that polypropylene capacitors can sound inherently better, but the change in internal losses changes the response of the filter, which is designed assuming the losses of the electrolytic component.
What usually happens when the low loss component is fitted is that the corners of the roll-off are sharpened, giving a peak in the combined response that can make the sound unpleasant in various ways depending on the crossover frequency. One way of getting round this is to wire a small resistor in series with the capacitor to approximate the original losses. I say approximate because the loss factor is a frequency dependent resistance. The actual value you need depends on the original capacitor loss factor and its capacitance value. The larger the value, the lower the resistance for a given loss factor.
The formula for the equivalent resistance is:
R = d / (2π fC)
where R = resistance in ohms, d = loss factor, f = frequency in Hz and C = capacitance in farads.
Loss factor is usually expressed as a percentage at 1kHz. For a "low-loss" electrolytic such as the values between 1μF and 20μF found in tweeter circuits, d is of the order of 0.025 (loss factor of 2.5%). For values in the hundreds of microfarads it may be of the order of 0.07 or 7%. Typically therefore a good electrolytic capacitor of 5μF would have an equivalent series resistance of 0.8Ω. If the capacitor has a much larger resistor in series with it anyway, it's probably not worth altering.
The same argument applies if you substitute a cored inductor with an air core type. Always try to duplicate the DC resistance as well as the inductance. Sometimes inductors are deliberately wound with relatively fine wire to give a certain resistance to add damping to the circuit. Iron dust cores (sometimes called P-cores) have higher losses at higher frequencies than at lower, due to eddy current effects. Substitution of an air core, even of the same DC resistance, may give a steeper ultimate slope in the stop band that can alter the phase relationship between the two drivers. That can also mess up the overall response and skew the optimum listening angle a little.
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As you can see, it's a potential minefield and difficult to get the optimum result without proper measuring facilities. Adjustment just by ear tends to give good results on limited programme material and you can usually come across some other piece that sounds less than acceptable.
The ESR shouldn't change significantly and I'd use separate boards for easy plug and play swappage.