I haven't converted from 22 to MO99, but have talked to one of my suppliers that has a customer that did. One thing they did was change the seals on all their solenoid valves. They put the same type of seal in material wise, but apparently a seal that has been exposed to 22 will fail when then exposed to MO99-some kind of chemical reaction takes place. They kept mineral oil as a lubricant, but initially had return problems. They solved that by adding a bit of POE (how much I don't know). Price wise where I'm from, MO99 is almost half the price of 22.
You should check out DuPont's site-they have a tech manual on retrofit guidelines for MO99 that covers a lot of topics.