Hello Abhishek,There is no change of board as such after 12th.JEE Mains is conducted every year for admission into NITs,IIITs and other GFTIs.The candidates who qualify JEE Mains are allowed to write JEE Advanced for admission into IITs.

So anyone can write JEE Mains irrespective of their Intermediate Boards but they should satisfy the eligibility criteria laid down by them(

https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp&ved=2ahUKEwiw4YieopjiAhUmS48KHXtuBdgQFjAKegQIAxAB&usg=AOvVaw0Ddf__yVIUe0laa-JjEit-&ampcf=1 )

So even if you change your board after 10th class,there will be no problem but you must have completed your class 12 before the admission.

Hope it helps you.