Incomplete list & in no particular order:
There are no Arabian characters on The Simpsons.
They was no episode of The Simpsons with Chelsea. Chelsea FC simply were represented as Simpsons characters as promotion for the Simpsons 25th anniversary.
There is about 100. You can go onto www.wikipedia.com and type in "list of the Simpsons characters" and you will find all of them, even guest characters
BartHomerMargeLisaMaggieNedKrustySeymoreEdnaMilhouse
No they are not. They are fictional cartoon characters.
the minor characters are x-ray
There was a Celebrity Version of Sesame Street's Monster in the Mirror on YouTube. Some of the Sesame Street characters appeared on The Simpsons, including Elmo. There was also a part of one of the christmas episodes of The Simpsons where the Simpsons characters are puppets.
The same as they are today.
Major
No doubt Bart or Homer.
Dan is the voice of the very popular voice of the simpsons Homer, Abe and many other simpsons characters.
Changed how? literally change out the characters with new characters or physical characteristics changed? Neither were necessary.