To change the value of 'advance_amount' of 'neworder' table with the following condition - 1.
Yours is not a case of nested CASEs, if you pardon the pun.
The only way your normal CASE could break is if it had more than 10 conditions and was executed against a linked server, which is why Aaron has asked you about it.
You cannot use an update query to add new records to a database, or to delete records from a database.
To add new records to a database you use an append query, and to delete whole records from a database you use a delete query.
Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.