Hello Aspirant,

Actually Diploma holders are not allowed for JEE Main admissions but they are allowed for JEE Advanced. So they can write the JEE Main exam just to qualify for JEE Advanced.So if you plan to write the exam after you complete the diploma, you wont be eligible. Instead you will be eligible to go for lateral entry as diploma holders go directly to the 2nd year of engineering. 

Hope this asnwers you well. Good Luck.