Can i apply for premium I have my receipt nbr

I applied for h1b 2013 masters quota in regular processing.,But now i am planning to change it to premium processing can i do that now. I received my receipt number. One more thing i dont have a client letter .if i upgrade it to premium will it be a problem without the client letter?

Yes, you can upgrade it to PP…Talk to your attorney.
If an RFE is issued for client letter after upgrading to PP, then attorney can respond back with Client letter.

“You” cannot upgrade. Your attorney needs to be involved. You are allowed to pay for the processing, but cannot do anything on your own behind their back.

If there is no client letter, whether its premium or regular, if uscis feels the petition is not qualifying the employer employee rule, it will be suspended.