Hello,
Yes,you can change your branch from. Mechanical to petroleum after diploma if you feel can do better in petroleum branch then go for it.
But it also Vary college to college, some colleges do allow some not.
And another option to change your branch is during your masters or PG course after Completing B. Tech in any stream  you can go for GATE  exam and can take admission in any branch you want.
All the best!!

JNEC Aurangabad

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