OP: I think you're misunderstanding the point of a medical waiver for change. The medical waiver is for situations where you're unable to fly for a medical reason, not unable to do an activity, such as ski at the destination. From United's website, you are required to provide documentation for this medical waiver:
"Illness situations require a letter (on letterhead) from a licensed physician confirming that travel was not recommended due to the customer's illness."
As such, you do not qualify for a medical waiver of the change fee from UA.

Now, I don't want to sound hard on you, but it's clear you're a somewhat inexperienced traveler and also inexperienced with insurance/credit products. Travel insurance is all about the fine print. You must read it carefully to figure out what is included/excluded. Perhaps a knee injury that prevents skiing would be covered under some travel insurance policies, although I don't know if it's very likely. In particular, you should pay attention to exclusions for "pre-existing conditions", as this is one of the main reasons that travel insurance policies end up being worthless.

Ok...now the preachy bit.
This board has been very helpful for me in learning my way around air travel as I progressed from a very poor undergraduate to a slightly less poor Ph.D. student to a now above-the-poverty-line-but-still-feel poor postdoc. Maximizing successful flying while minimizing costs is something I've learned from this board. There are indeed rules, and many flyertalkers may try to skirt them a bit from time to time, but ultimately, I think most of us have a pretty good sense of fair play. We expect UA to treat us fairly, and we try to hold up our end of the bargain. I'd encourage you to take a step back and try to absorb some of the advice in this thread without getting mad that it contradicts the result that you'd like.