Not normally.
Ed allowed a batch of changes last year as a one-off but I doubt it'll happen again for some time.
Some people just start a new account with a new name, but your history of Grenhelm posts would remain under that name
There are too many names to mention.
You can just sign out and then instead of signing back in just select the Register option and start as Grenhelm 2 or whatever you want.
Can you tell us the reason as there may be alternative approaches
You will need to use a new email address, probably a good idea to set up a new one with say Hotmail or Google mail. There are other email providers out there :-)