Is it possible to upgrade my current free webhosting plan to paid webhosting plan?

Yes, if you are using of free web hosting services of any company you can upgrade anytime from free to their paid hosting solutions. You can contact them on live chat, submit a ticket or email.

Yes, most of the webhosts offer you to get an upgrade to the paid plan but this may involve little downtime while they transfer you to the paid service hosting server.

Yes, this is generally possible if there are paid plans higher/better in terms of offering than your free hosting plan.

Of course you can upgrade the plans. Ask your web hosting provider to help you in this case.

Yes, you can upgrade the plans. Check with the hosting provider for upgrade options.