Just do it the simple way and download the Level 1 NPCs mod (v1, not beta7). That allows you to change NPC classes and kits - even to illegal multi-classed combinations. Want Minsc to be a Fighter/Wild Mage/Thief or a Kensai/Mage/Thief? You can do it.
Want Jaheira as a Ranger/Priest of Helm? Easy. Want Imoen to be a Wizard Slayer/Wild Mage/Swashbuckler?
Using Shadowkeeper, you have to change a lot more than just simply putting the level back to 1.
You have to reset the hit points, saving throws, THAC0, AC, proficiencies, innate abilities, spell slots, thieving skills, lore, and maybe more back to what a level 1 character would be. This is a lot of work, especially if you don't really know what you are doing.
I too would recommend the level 1 npc mod since it does all this stuff automatically.