So you cancelled or UA cancelled? If you cancelled then you have a flight credit. If UA cancelled the flight then they owe you a full refund or you can alternatively get an ETC.
Trying to convert an already accepted 'flight credit' (you must have done that) to an ETC is not easy and UA may rightfully refuse to do so. There is no such thing as a refundable ticket that can be converted to an ETC. You can use an ETC to buy a refundable ticket that can lead to extra flexibility, not the other way around.
Using a flight credit from an existing ticket will have the same restrictions as the original ticket, however, if you book now they will give you extra flexibility.
Short of calling and pleading your case there is nothing you can do.