C.W.
Just talked to our pediatrician about this at my son's 4 year old well-visit. Her take - keep on the pull-up, he's got a long while before this is a problem (in fact, our doc said, reassess again at 4.5...and even sometimes kids can take as long as to age 7 to be fully night-trained). Her reasoning was that if he's waking up SOAKED, it means his body/brain is not yet mature enough to make the connection while sleeping for such a long time (naps have been ok. I figured once we had no wetting during naps established, we could then address the longer stretches). Anyway, this is not a situation that should be pushed (just like potty training in general). In fact, we still use diapers at night due to their absorbency. A pullup is not made to be as absorbent and I was cleaning up A LOT more overnight leaks. Another friend of mine did the same as her daughter (over age 4.5) was sleeping through being wet and having it sit next to her skin for so long they were dealing with some major skin irritation and chafing (again, pullups do not withdraw the moisture away from the skin like diapers do).
More and more my son wakes up drier or comes to find me in the middle of the night for help to use the potty. When he starts to do this more regularly, we will consider making the switch (which our doc advised....limit fluids after dinner, wake up child to use potty before we go to bed, etc.).