I have an Ionics Pro 5 and although I am very happy with the system I have always resented paying £65 every few months to replace my colour change DI vessel.
I have come up with a way which is so simple to replace your resin. Firstly order a 25L bag of Resin. You are best not to use colour change as I hear it doesn't last as long. I have Rohm and Haas resin but I hear Dowex 50 is also very good. I got my Resin from Reach and clean for about £80 including delivery and VAT and will expect it to last about 2 years
Then get your old DI vessel and drill a hole the size of a 5p coin in the clear plastic tube about 4 inches away from the end cap.
Then attach your garden hose to the end cap on the other end of the vessel. Turn on the tap and flush the old resin out. Best to put the vessel in a bucket as the resin will leave a right mess in your garden!!
When all resin is out lay the vessel on a plastic sheet, put on your plastic disposable gloves and start pushing the resin in. This took me about an hour to do. The best way I found to put it in was to pile a handfull of resin on top of the drilled hole and then gently rub it in throught the hole. Alot of the resin will fall over the sides onto the plastic sheet but because the sheet is clean you can just pick it back up and rub it in again.
When the vessel is full (but dont over fill it as the water will need to have room to move the resin around) get 3 rubber party balloons, fold the mouth ends into the body of the balloon and put it over the hole. As the balloons are soft rubber, they can easily be moulded to the shape of the hole in the tube.
Then get a jubilee clip big enough to go around the circumference of the vessel and put it over the balloons to hold them tightly in place, and tighten.
put it on your system and test. like me, I needed to take the jubillee clip off as there was a little leak but 2nd time it worked perfectly. I have now been working for a full week now and havent had a drip yet (Im not one to take a risk so I have spare balloons in the van just in case!)