It is possible, but you might as well pay full retail price for the phone. Like awesomenessiq said, plus the new line activation fee. If you have the money to dish out full retail might as well just avoid that and pay full retail.
I think it depends on the time frame you try to cancel the line. They may still charge you the full retail price if you don't return the phone. If that is the case what you would really end up paying is the ETF, the difference between the full retail and what you already paid to originally purchase and the $35 activation fee. Not completely sure about this but I do recall posts about situations like that in the past.
In addition to what the others said, you'll also want to double check the early termination fee itself.
The ETF for higher cost cell phones used to be a higher dollar amount and it was pro-rated at the beginning of the contract. The fee reduced as each month went by.
Lower cost cell phones used to have lower ETFs. Just be sure if you decide to go this route, that you have something in writing that says what your ETF will be if you cancel your contract right away.
Good luck with your decision!