I'm a little confused - you said "downgrade plan to just one phone" - does that mean you are on a family plan with more than one line right now? Each line has it's own cotnract and end date....and if you agreed to a 2 year contract for both of those lines, then cancelling one line would involve an ETF if you had not fulfilled the two years agreed to in the beginning. The ETF is $175 less $5 for each month completed of the 2 year agreement. If your current contract ends in December, that means 18 months completed. $175 - ($5x18) = $85 ETF fee.
If you are the primary number on the account, you can log in to MyVerizon and see when each line's contract ends.
You can also change your plan and reduce minutes, eliminate texting, etc, to try and cut costs. That will not change your contract end date at all.