Personally, one of my (long standing) gripes is that *I* don't have a 5 point. I grew up with my uncles racing... so all of our cars were converted into 5point harness belts.
My son outgrew the 5point seats when he was still a toddler. (Of course, he'd outgrown the infant seat by 6months). THE MOST IMPORTANT THING about a 5point is that the straps have to terminate ABOVE the shoulders (or the seat itself can snap your spine in 2 in an accident). At the time, they didn't have any 5points to fit him... so he was in an adj seatbelt booster by 2. (Ours DID have LATCH points, however).
The transition was fine. No head flopping.... and he learned that the belt MUST stay on his "hard parts" ((I would press on his sternum and hips))... and after about a week of "Hard parts?" while he adjusted his belt back from where he'd pushed it, they've been there ever since. I also pushed hard on his "hard parts" and his "soft parts" (aka hips & sternum vs stomach and neck) so he could feel a little of what it would be like if the belt was in the wrong place if we stopped suddenly. Not enough to hurt, but enough for the glug/gasp thing.
My neice and nephew used to be notorious about squirming out of their belts. My SIL started doing the "hard parts" bit, and they quit squirming. Of course, that won't work for all kids... but the 3 of ours tend to be the types who will do something if the reason makes sense to THEM.
Anyhow... he outgrew the positioned seatbelt between 3&4 (the highest setting had the belt half and half on hard and soft parts... and his head was getting higher than the headrest)... so we switched to a straight booster, which has worked grand.
I STILL want a shelby with 5point belts though. Or a hybrid with 5point racing belts. Sooooooome Daaaaaay Over the Rainbow.....