Can i send f 103. i haven't recd conversion ltr.

Dear sir/madam, i've sent application to CA institute for IPCC conversion into intermediate (IPC) course. I have not received my conversion cum registration certificate yet. My friends have told that i can register for article with existing registration number. But i'm thinking i can register for article after getting conversion letter from institute. Can i send form 103 immediately or i have wait untill the conversion letter receiving date? Kindly advice me.