upgrading crossover components on speakers

Has anyone ever dealt with tweaking perfectly good orginal crossover components to get better performance. I have to replace a broken capacitor on a crossover with something other than the original. I will be replacing an original Polyester capacitor with a Polypropylene one. Some people have even suggested that I replace all of the Polyester capacitors on each network with Polypropylene ones. Anyone have any experience with such tweaks?

Also, could I not create identical crossover networks to play with by just building a new one with components that have the same values as the existing ones? I mean there is no magic to the way the parts are layed out on the PCB, right? As long as the signal path is correct, right?

Regarding the caps, for the sake of symmetry the "other speaker" crossover's cap should be made the same as the one that you're repairing. You could change just those 2 caps & then listen to determine if you notice any improvements. If you do like the change then maybe try replacing some others. Alternatively the remaining caps in the Hi freq. crossover section could also benefit from installation of small bypass caps such as 0.1 uF's soldered across them, rather than wholesale replacements.
Regarding building complete new units, "same component values" would include inductive & capacitive Q values, not simply the L & C measurements. You could end up assembling a completed unit that appears identical but does not even sound close to identical. Regarding layout: pay attention to inductor architecture & alignment; they can cross-couple via stray magnetic fields. Just replicate what the designer has already figured out.
What kind of speaker are you tweaking? If I were you I would search at the audioasylum.com speaker and tweaking page and the diyaudio.com site and see if anyone else has changed the caps on the speaker you have.

A lot of folk I respect, like Lynn Olson, think that the the tweeter cap is the most important passive component in the entire system! (Read Olson's article "Evolution of a crossover" at the www.aloha-audio.com site. I also suggest you use the best cap there is and both speakrs need the same tweak. Take a look at the Northcreekmusic.com site.- one of the best passive XO available retail and lots of good info and aplace to buy caps.

I think, generally speaking, changing caps in the signal path to the tweeter, is one of the best tweeks there is.

Also, there is a "magic way" stuff is laid out! All of it interacts. (An oversimplification) Magnetic fields, for example, run at specific angles to the current and placement of some components chokes/coils can be crucial to avoid crosstalk ect.

If you just replace the caps shouldn't be an issue.

Bundus is correct when he says you will not know what you have until you hear it even though the design appears same. That is why I think it's great if you can find someone who has tweaked your speaker already. This stuff goes on all the time. Having looked at a lot of fairly expensive speakers it is sad how cheap the XO components are in most.

Sincerely, I remain
For the sake of being complete, I never had to replace the caps on my crossover. It turned out there was a crappy soldering place in my crossover. It would variously work/not work depending on the temperature (figures). I eventually bought a gun and shot^H^H^H^Hsoldered it myself. It didn't take me 3 years to figure this out though. It just took 3 years to type it up.