No. It is not possible. They will allow a change from pen & paper to CBT but not the other way round.

By the way why do you want to change? I would say you should go for the CBT. There is a lot of myth going about JEE Main pen and paper advantages. Believe me thats is not the case. When students go for BITSAT or VITEEE, they dont have a choice of pen & paper based mode and all. Most exams in the future will be online.

With CBT, you have timer, you can review your questions if you mark them like that, moving between sections is easy - there are advantages. Trust me I know many kids who have attempted the CBT and gone on to score well. The mode of exam wont matter. Your preparation will. So dont worry about this aspect and dont listen to anyone telling you that pen & paper is best.