Newbie Question

I’m sure this has been asked many times on other D&D forums, but I’ve never really played basic D&D before, only had a brief foray into AD&D… My question is this, if you play a non-human and have a level limit (for example, 10 for an Elf), what happens once you reach level 10 in a game? Also, does ACKS allow for people who want to change class, multi-class or whatever you want to call it?

You can keep playing a character that has reached its level limit - 10 for an elf, or 14 or a human - and strive for other rewards. Not being able to level up doesn’t mean you can’t amass wealth, grow in power in the world, pursue quests, etc.
ACKS doesn’t provide rules for changing class and multi-classing, but that isn’t to say it’s not allowed - every campaign is a law unto itself! You could adapt rules for it from another game, or make up your own. Also, the custom class creation rules in the Player’s Companion do a good job of making a class that feels like a multi-classed character - my son used 'em to make a fighter/magic-user/thief that we were both very happy with.