Does anyone do speaker mods?

There is a difference between upgrading and redesigning. I take upgrading to me matching values of existing components and replacing them with better ones.

There are many avenues for crossover redisign. the forums at HT guide have many members that can do it. In addition, Meniscus audio and madisound do it for a fee.

S7horton, you're right, there's a difference. Just to clarify, my upgrading only consists of replacing selected parts with the same values, just much better parts. I choose the parts based on what the customer wants and how far they want to take things. I also tell them, that even the entry Auricap, that is pretty much the cheapest in the chain, is going to offer a noticeable sonic improvement. I talk and explain the entire process over the phone all the way through from start to finish.

*****NOT KNOWN BUT VERY, VERY IMPORTANT,*****
*****YOU DON'T NEED TO SHIP YOUR SPEAKERS*****

That makes a huge difference to me. Unlike most mods or upgrades, upgrading speaker xovers only require the shipment of the xovers themselves, which makes things really easy.

Speaker design is so complicated and so many factors are tuned in that changing a driver could make a mess out of everything, If you change the tweeter and it has more sensibility than the older one, higher frequencies will be louder and the speaker will shout at you, you would need to add some padding in the Xover changing resistor values, this requires a long time of listening and measuring and readjusting, you will probably need to buy 5 or 6 resistors to have the different values to change them around and cheap resistors sound horrible. A better tweeter will probably be more efficient.
Changing a woofer is worst because the cabinet is designed from the parameters of the woofer (small thiel factors) so you could get a better woofer and end up with worst sounding speakers.
Upgrading selected parts of the Xover is probably a good idea since caps could cost much more than the drivers, my guess is a serious loudspeaker company tunes different capacitors to the sound of their drivers, but I am sure some will use the cheapest Xover parts since we dont really get to see them.

Speaking as someone who builds speakers, I wouldn't say that removing the crossovers and sending them somewhere is very simple. Especially for those who have never opened up their speakers.

Jsadurni, I disagree that a better tweeter will be more efficient. It is completely driver dependent. Some are efficient, some are not. In addition to needing to potentially pad the tweeter (which is a good point) the crossover point could need to change. Which means complete crossover redesign on the tweeter side assuming it is a parallel crossover. If it is a series crossover, good luck. Some tweeters can cross low, some can not. You would have to know the optimal crossover point and find a driver that met those specifications. No simple task.

S7,

For the majority of speakers, we're talking about a few screws and a positive and negative wire to each speaker piece (tweeter, mid, woofer, etc.)

I don't think that's very difficult. You just have to make sure that you label the positive and negative wires and to which speaker piece they go to. Many xover PCB's have the labels on the PCB as well (to which speaker piece they go to.

Again, not very difficult. Pretty straight forward if you ask me. Heck, my 12yr and 14yr old sons understand it and have helped me from time to time.