If you already have a Web application created using Classic Mode Authentication or How to convert Web application from Classic Mode authentication to Claims based Authentication, Then You don’t have need to delete that web application. You can convert that web application from “classic mode authentication” to “claims based authentication”. However this can only be done using PowerShell and it’s an irreversible process. Follow PowerShell commands to convert the web application from Classic Mode Authentication to Claims based Authentication:
Step 1: Open “SharePoint 2010 Management Shell”
Goto Start=> Clcik All Programs => Expand Microsoft SharePoint 2010 Products => Click on SharePoint 2010 Management Shell.
Step 2: Type Following Command
Syntax: $App = get-spwebapplication “URL”
Example: $App = get-spwebapplication “http://b2b-shp10-dev:12000/”
Step 3: After That Type Following Command one By one
Syntax: $app.useclaimsauthentication = “True”
ScreenShot:It Should be look like that.