Autism is something you are born with. It is simply who you are.

Support needs can change over time. You can need less help because you learn better coping strategies and have a stable environment or you can need more as you get older. It is not fixed.

Support needs are denoted in level because that is what system like schools and the like need. They don't really map to reality. Like for example a autistic person can have really bad sensory issues, being really sensitive to sounds, restricted diet and the like but decent social skill. Another autistic person might not have any sensory issues but really struggle with social stuff. Who needs more help? They need different kinds of help.