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You should use identity to add or remove a claim. Try this to add a claim.

var user = User as ClaimsPrincipal;
var identity = user.Identity as ClaimsIdentity;
identity.AddClaim(new Claim(ClaimTypes.Role, "somenewrole"));
To remove a claim,

var user = User as ClaimsPrincipal;
var identity = user.Identity as ClaimsIdentity;
var claim = (from c in user.Claims
                         where c.Value == "somenewrole"
                         select c).Single();
identity.RemoveClaim(claim);
BTW, it is better to use User from your controller instead of HttpContext.Current.User.