Officially the change fee is only waived if you are upgrading to a higher cabin. Unofficially, I've had some luck getting agents to waive the change fee to upfare within Economy, but in general courtesy fee waivers seem a lot harder to get than they were 6 months ago so I would expect more pushback on that now than I would have before. It is not in United's best interest to let you buy a discount ticket then upfare to a fully flexible ticket to avoid paying a change fee.