Thanks MEP, you saved me a lot of trial and error.
Everyone was right. It took me 1 1/2 days to do 2-21"x62" tanks. FWIW....here are several of the things I learned. I hope it may save you some time....
Wood Brothers had the resin (8% Crosslink) at less than $50 per cuft. Locally, they wanted $125! They also have the gravel if you want it. My units required 125# of gravel and 7 cuft. of resin per unit.
I purchased a special water softener funnel for less than $15. While you can make your own out of a bucket or similar....this thing was worth it's weight in gold since it was heavy duty, fit perfectly, and was elliptical (instead of round) in shape. This became a factor, since the new resin does not really "pour". It had to be shaken in using a scoop....and this is a chore when you have to put 7 cuft. of material in a 4 inch hole with a 1 1/2 PVC pipe in the middle.
Speaking of the PVC pipe, I used new hubs and laterals (also provided by Wood Bros.). The old ones looked OK, however, the laterals have very fine slits in them for the water to enter. A lot of the slits were clogged with fine bits of old resin....and I didn't want to take a chance on installing $$$$$ worth of resin and still have flow problems. A new hub and lateral cost less than $40. The 1 1/2" PVC pipe came from Lowes.
I used an old ARO diaphragm pump to pump out the old water and resin. Per MEP's suggestion, I put a garden hose down the center distribution tube, and pumped out the water/resin mixture into 5 gallon buckets in a SS bay via a long poly hose in a SS bay. A helper would fill the bucket, let the mixture settle, pour off the water at the top, and then dump the bucket in the dumpster when full of resin. (My dumpster is 40% full of orange resin. I wonder if the trash man will have any thoughts about seeing this mess???)
When the unit was pumped down to the gravel, it could be removed with a hand truck to a SS bay where the unit was inverted and cleaned. It was very heavy, but doable.
MEP's idea of installing a union was a godsend. These units are so big and heavy that there would be NO WAY you could get the heads off without getting the connecting pipes out of the way.
The guy at Wood Bros. suggested that I have 2 replacement o-rings for my Fleck 2900 softener head just in case. The o-rings were for the head to distribution tube, and also the head to the fiberglass tank. I think they cost $5 total per unit. I always use o-ring silicone grease around the car wash, and it worked well here also.
I had my wife, with her small hands, assemble the hub and laterals. Although the instructions said to assemble the hub/laterals first, and then cement the PVC distribution tube....we chose to cement the PVC pipe first, and then assemble the laterals. The PVC pipe gave us "something" to have a hold of, and it worked better this way. We did this in the SS bay while it was on it's side in case something got dropped inside.
Don't forget to tape over the PVC pipe when refilling the gravel/resin. I used Gorilla tape...and it was great. The last thing I wanted was for the tape to fail with a funnel full of material ready to fall inside.
As someone here said, it's not hard, just time consuming. It took 4 hours to put the resin back into both of the tanks. Your mileage may vary.
This forum has saved me a lot of time and money. I hope this information helps you also.