Vodafone processes requests for migration of post paid to pre paid, But it is tricky process.
The call center agents and the people at vodafone store would say they can not convert from post paid to pre-paid.
I raised a complaint to Nodal Officer e-mail id.
[Got it from Contact us page on vodafone website].
The nodal officer sent an e-mail saying that they can process the migration from post paid to pre paid and asked to carry the e-mail that he sent to vodafone store.
I submitted the e-mail to vodafone store and then they have converted my post paid to prepaid.
I would suggest to contact nodal officer if you are looking for migration of post paid to pre paid.