I have sucessfully used the following script to change the dnn username (by going to [b:2t174ohg]Host[/b:2t174ohg] -> [b:2t174ohg]SQL[/b:2t174ohg]):

[color=#000000:2t174ohg][code:2t174ohg]
update users set username=’newusername’ where username=’oldusername’
update aspnet_Users set username=’newusername’ ,loweredusername=’newusername’ where username=’oldusername'[/code:2t174ohg][/color:2t174ohg]

[color=#000000:2t174ohg]For each reference of [COLOR=#339966]’newusername’ [/color:2t174ohg]and ‘[/COLOR][color=#000000:2t174ohg][COLOR=#339966]’oldusername'[/color:2t174ohg][/COLOR][color=#000000:2t174ohg], substitute the names you wish to alter.[/color:2t174ohg]
[color=#000000:2t174ohg]Check the box titled [b:2t174ohg]Run as Script [/b:2t174ohg]next to the [b:2t174ohg]Execute [/b:2t174ohg]link below the SQL window.[/color:2t174ohg]