To clarify, your only choice is to pay for a ticket change to BE. This will require the difference in cash between the BE and what you paid.

If this is a RT, it gets a bit better for you wrt price. The RT tickets dpt Sunday with 1 week stay are in the 3.2 - 4K range. If you only need the outbound in BE, then it might "only" cost about $1.5-$2K.

If you are flying one way, you can also "cheat" and buy the RT biz at cheaper than what it would take replace the existing one way. If you do such, put that unused return sometime out in the future when you might find a use.

I know you did not not want any other suggestions, but a strong sleeping pill might not be that bad in this case. Flying while sick is miserable.