Yeah, I've posted on subjects like this before.
But here is a better recap.
The best way to edit any character is to set his/her level to 0 and remove all "affects", weapon proficiencies, innate abilities, and thieving skill points. Change the class and kit (if applicable), and select a new avatar (if you desire). You don't need to change saving throws, THAC0, or the number of spells the character can memorize--those will be adjusted automatically. Ideally, a level 0 character should start with 0 hit points, but it doesn't work in practice, so I set it to 1 or 2 and then remove those extra hit points later. I normally set the character's experience to 0 and level him/her up manually with CLUAConsole commands.
When you load the new saved game, the character will level up to 1st level automatically. At that point, he/she will acquire all of the abilities, proficiency points, spell slots, thieving skills, saving throws, THAC0, etc. that a 1st level character of that class should have. After you give him/her the correct amount of experience, he/she will level up to the appropriate level. If he/she started with 1 or 2 extra hit points at 0 level, you can remove them with Shadowkeeper. If you do all that, the character should be "perfectly" in compliance with the rules for the new class.
Some NPCs require special treatment if you edit them. For example, if you want to edit Imoen while you're still in Irenicus's dungeon, she is programmed to run away if she only has 1 hit point, so she needs more hit points than that at 0 level. Characters such as Yoshimo carry special items that they cannot use if you change their class. If you intend to dual-class a character such as Minsc, you have to change his stats to meet the dual-class requirements. Haer'Dalis isn't really a Blade (he's a specially modified Tiefling), but it doesn't seem to hurt anything if you change his class. Anomen got shortchanged on proficiency points when he was created (he should have four more proficiency points), and other NPCs have the incorrect number of thieving points and things like that, so editing will actually make them more likely to be in compliance with the rules of the game.
Dual-classing sometimes creates some minor difficulties. (It involves a flag that cannot be changed with Shadowkeeper; you have to use a program like Near Infinity.) If you run into any problems, just ask for help. I like to make Yoshimo a 7th Level Kensai dualled to a Thief (it's perfect for him), but I want him to have the avatar of a Monk. If you change his avatar to a Monk at level 0, the game automatically "corrects" it and changes it to the avatar of a Fighter, so you have to go back into Shadowkeeper and change it again. This also applies to Mages, who have a tendency to slip back into Thief or Fighter gear. There is a mod that has the option to make the armor animations "consistent"--it automatically changes your character's avatar every time you change armor. (I can't remember exactly which mod it is, but it's one of the popular tweak packs.) I don't recommend enabling that feature if you want to use a custom avatar.