As mentioned, invol re-routes are typically even-exchange. If you want a refund for this type of travel, would you then be ok with UA to also tell you that you owe $X or Y miles when the invol reroute flight is more expensive than you’ve paid.

It typically, at that point, is just by same cabin. If there was a situation where you were in F and took Y to get home sooner, you’d get a partial refund, but even then its typically less than one might expect.