That sounds like a lot of work. You'd be re-inventing a lot of the monsters in use, mind you, as they make original creations for the APs. Some of the later encounters, I believe, also assume high-level tactics.
Someone mentioned Council of Thieves (I think) stopped around level 11.
I don't know if you'd mind seriously increasing their magic item count and power. If they have +5 weapons, a lower level party could do more damage to higher level foes. The challenge would be saving throws and resistances, but items would help for that. Also scrolls of basic protections, then let them concentrate on different spells.