You're right, that is probably the answer. I need to figure out a conversion formula for options on later futures expiries so that I can create a consistent curve going out to 2021. My expience with options on rates derivatives or even rates futures themselves is limited so I'll have to hunt around for the correct formula.
I think I am

You need to know the volatility difference between the futures. So in interest rate markets... it's about duration etc. I'm not an expert in bonds etc, but keep in mind... a longer date bond moves more than a short term one. So you need to figure out the relationships between say the 2 year and 5 year... etc... that will be your starting point.