That is not possible. You can simply buy a prepaid card and start using that. You would, however, lose your number if the prepaid card is also with O2. If you go with a different provider you can take your number with you.

Hello @pappu and welcome to our o2 community :-)

There are often reasons why a change to a prepaid solution is desired. If you just want to be flexible, have you already heard of the possibility to change to a Flex-tariff? This tarifss only have a minimum contract time of one month and can be canceled at a months notice. Maybe this could be a solution?

Regards,

Lars