Can my unit override my ETP ?

Nope. Not without a real good reason to do so. Your unit can extend or expedite your departure pretty much as they see fit. If you asked, what was the reason they gave for changing it?

Dear Jeremy, I pray that you are successful at finding the answers to your questions.

Are you flying to your next duty station or driving? If you're flying, talk to your ATO/SATO and find out what the tolerance (+/-) is that they will book tickets from the approved ETP dates. Most likely they will only book tickets +/- 3 or 5 days from the approved ETP date. If you're driving then make sure whatever day you leave still allows your the number of required PCS travel days in order to report on time (or by the NLT date). As long as they let you leave with sufficient travel time to report on time, I'm not sure there's much you can do.

On another note though, usually you have to submit your PCS leave forms with your Levy packet in order to get your PCS orders cut, so you should have already had an approved leave form to depart on the date you wanted to depart... so are they cancelling your leave?