Unexpectedly named by Queen, one changing for the better?

• Bill modifier, e.g
• Change maker
• Editor, at times
• One proposing a rider to a bill
• Copyreader, e.g
• One who tacks riders on bills
• Constitution tweaker
• Person who makes change

Wiktionary Word definitions in Wiktionary
n. One who amends.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Amender \A*mend"er\, n. One who amends.