aspnet membership username PK vs FK

Hello,

We are using .net's membership provider to create our application. And our existing tables had the username as varchar(20)

However I noticed in the aspnet_users table, the username is nvarchar(256). In order to create PK vs FK relationships, does this mean we will have to change every instance of a username to nvarchar(256) ?

 lastly, why is MS using nvarchar, I thought it was better to use varchar ?

thanks

 

 

 
...