Has anyone hired cyrix7x86 on Ebay to "upgrade" their crossovers?

I exchanged a few messages with him but ended up building entirely new XOs myself. 

My other question is if an "upgrade" translates to a noticeable difference in the quality of sound being produced by the speaker.

In my case, I experienced a substantial improvement in SQ. Greatly improved resolution and transparency being the most obvious benefits. But caution is advised as it's also possible to make things worse. One of my precautions was to make sure I didn't deviate from the DC resistance of the circuits.

I imagine that the component parts of a crossover do not deteriorate over time - or am I mistaken? 

Most capacitors, resistors and coils will last for many, many years (well, tortured use such as high SPLs *could* cook resistors). One exception is electrolytic capacitors which can drift with time. If you have relatively inexpensive speakers or if the crossover includes caps with high value capacitance (eg, >50 uF) chances are electrolytics are in there. From my reading, these typically last 10-20 years. They will continue to work but speaker performance may decline.