Here are your answers
1) As I don't have backup of certificate and .p12, should I need to raise a request for new certificate from my keychain and proceed with that?
Don't worry, when you like to give new update for your application, create new .p12 file and use it. Certificates are used to basically authenticate your machine with developer account.
2) If so, will users can be able to upgrade the existing app from the appstore?
No problem for users, as app store distribution provisioning profile works very different from developer provisioning profile, so no user needs to update.
Just for your info: the signing files for Android are very important, not for iPhone application. For Android, if signing keys are lost, you cannot update apps, whereas for iPhone you can create new certificates and update your apps.