Hi, Thank you @agarwalc for your recommendation.
Yes, the plugin 2Customer - Magento Guest To Registered Customer will certainly do your job very well and give you some more features like Automatically Converting a Guest Customer to Registered Customer, Automatic Assigning of Orders to Customer accounts of Customers who have done the Checkout as Guests etc.
Complete List of Features
Convert Guest Checkout Customers to Registered Customers Automatically After Order is Placed
Alternatively, give your customers an option to convert to Registered Customer in 1 Click on Order Success Page
Automatically Push a Welcome Email with Username(their Email Address) and System Generated Password to your Converted Customers. You can also disable it if you wish to
Careful handling of Existing Customer Accounts. If an account is already existing with the same email address given on checkout, the order will be merged with the existing account automatically
Separate Section in Magento Admin to Manually Convert Each New Guest Customer to Registered Customer. Option to choose the Customer Group also Available
Easy 2 Minute Installation and 1 Minute Configuration
Works with Magento CE 1.6.x and 1.7.x
Free Installation and Bug Fix Support
Free Updates for Life
Detailed Documentation Included with the Extension
Price is $30 only!