I've often done SPD pedals with just an open end wrench and a 1/4" drive socket. You use the open end on the adjustment nut and the socket on the lock nut (I'm not giving sizes here as there are some that differ by a mm, or two. Just measure and it will be obvious if it's correct, or not). It's nice to have the nested socket special tool (mine is MIA), but you can go without it. If I remember correctly, you will need 22 3/32" loose balls for each pedal. A good pair of tweezers, plenty of time, and patience, is what you need to do these pedals. Put some grease in a small cup, or whatever, and put the loose balls in there and get some grease on them. If you don't, they will be all over the place. Otherwise, it's no different a job from doing an adjustable cup BB or threaded headset. Nothing tricky going on here. You WILL need the special Shimano PD-40 for the pedals that have the splined axle lock nut (the one next to the pedal wrench flats, not the bearing lock nut). It's a shame as they used to include this tool (it's plastic) with new sets of pedals, but now you have to buy it. Fortunately, it's less than $3.

I would not even try to do this job without a vise and some axle vise jaws to hold the axle. Juggling cats would be much easier. I did a very long write up on this whole proceedure a few years ago on this forum, but I'll be damned if I can find it. A lot of the really old postings seem to be gone from the database. Too bad about that.

Not worth it for a shop to do it. This is pretty much a DIY'er job.

Almost all Shimano pedals - SPD and SPD-SL alike - have the same axle and bearing design. The one notable exeption are the DuraAce pedals which contain an additional set of roller bearings that sits between the two sets of ball bearings on the axle.