+1 - When in doubt, why not read the COC? The question then is whether the tariff for OP's ticket would have permitted the EVA rebook. One can imagine why not.

Needless to say, UA can, at its option, provide greater than the COC requires, but that's a gesture not a requirement.

What the arrangements are between UA & EVA is a speculation. What UA's internal employee policies are is also speculation (and not relevant).