Dear aspirant hope you are doing well..

If you want to become eligible for CDS  EXAM, then I want to inform you , those candidate who completed their ug degree in any discipline are eligible for CDS EXAM . To enter the Indian Military Academy you need to have a bachelor's degree in any discipline from a recognized university and your age should be in between 19 to 23 years.

So to become eligible, their is no need to switch your stream. If you are interested for doing PCM/SCIENCE in 12th then it's all your decision. Many others career opportunities are open  for you after doing science from 12th.

hope it's helpful for you

thank you