I'm far from an expert, but I have been studying about this too, over the last few months. According to what I've read, changes to ESR only come into play if you change your capacitors from electrolytic, to polypropylene or film caps. If replaced with electrolytic, the ESR values shouldn't shift enough to be audible. Mylar caps also exhibit a similar ESR value to electrolytic's. Some users like to upgrade with mylars in the tweeter circuit, and poly's for the mids/bass, to avoid excess brightness.
Now, if upgrading to poly caps, the ESR will be lower. How much lower is up for debate. It seems that a capacitor's ESR value shifts with frequency, and the value isn't exactly set. Though, typically, ESR only changes (drops) anywhere from 0.1 Ohm, to maybe no more than 0.5 Ohm, when upgrading to poly's. Not a huge shift. The ESR shift can be audible, but it may or may not be objectionable. It just depends if you like the treble a little brighter or not?
As a fix for the lower ESR, you can add (solder) a low value resistor in series (end to end) with the poly cap. You can try several values, if needed, to tweak the high end response to your liking. Use a decent resistor, with a least a 5 watt rating for the tweeter. Ten watts or better, if you like higher volume levels. Let the new caps break in for a week or two, to let them settle in, then if the tweeter is still too hot, start experimenting with adding a resistor.
Hope this helps?