Thanks for the kind words.
The full anti-freeze kit on the Mk 25 includes:
1. A composite piston with a mirror polished stem (standard on late production Mk 20's)
2. A plastic cover for the piston head with a ridge to prevent ice migration,
3. A soft rubber sleeve that covers the exposed portion of the piston stem and flexes to break off any accumulated ice when the piston sets back down on the seat (comes standard in all MK 20/25 annual service kits)
4. A teflon coated mainspring that resists ice accumulation (already standard on the Mk 20 T.I.S.)
5. Larger oval shaped holes in the swivel cap that allow more water exhcange in the ambient chamber (standard on late production Mk 20's)
6. Fins on the swivel cap to increase heat transfer (introduced after Mk 25 production was a year or two along so early MK 25's won't have them either)
The Mk 20 and Mk 25 transformation was an evolutionary process with several detail changes in each model. There was no clear defining line between them other than the addition of a adjustable seat retainer that allowed IP adjustment (and it's not worth the price of the upgrade.) So depending on when your Mk 20 was made, it may already have everything except the finned swivel cap.
The composite piston is worth having, especially if you have the mid production rounded brass tipped piston which do not lock up all that solid (they were never recalled as they never posed a safety issue, but the prescribed fix for a warrantied MK 20 with one that will not lock up is to replace it with a new one at no charge to the customer.) The piston will come with the cover and the set costs the dealer about $25
The swivel cap is worth upgrading if yours is a very early MK 20 with really small holes in the cap designed to used with a SPEC kit or a mid production Mk 20 with larger but still round holes. The large oval holes help a lot but the fins make no noticeable difference in my opinion so if you already have the large oval holes in yours, I'd pass on upgrading the cap. Dealer cost for a new cap is again about $25.
SP does not require any set price on parts so dealers can sell them at cost or make a profit on them. They do need to cover shipping costs and are entitled to make some profit, but should not be trying to gouge you too badly.
There also should be no extra labor charge for the upgrade if it is done as part of an annual service and other than the cap and piston, there are no other parts that are not already included in the annual service kit.
If you are doing cold water diving (below 50 degrees), be sure to have them lower the IP to 120-125 psi. It reduces the flow rate slightly and helps prevent freeze ups.
Scubapro sends free replacement parts to dealers to allow them to upgrade the yoke or DIN retainers on any Mk 20's that come in for annual service at no cost to the customer. The "new" universal retainer was introduced late in Mk 20 production, so it may or may not need to be replaced. SP policy is to replace them all however and then install a new plastic trim piece with 2 grooves to allow techs to easily tell them apart. (not that it is hard to tell the difference anyway, but SP has attorney's like everyone else.) The new trim piece looks amazingly like the one on my MK 17 and is no doubt the same part.
The only way cracks can develop even with the old style retainer is if it was badly over torqued to the point that the retainer flattened the sealing o-ring and bottomed out in the first stage body. This would place stress on the regulator body and could lead to cracks. If cracks are found the body needs to be replaced, but this should again be at no cost to the customer. The new retainer has a lip on it that ensures the retainer contacts the outside of the body and is stopped before it can totally flatten the o-ring and bottom out in the first stage body.
