To my knowledge they will offer you space on other flights, since it is not your choice, but the operating carrier who changed the flight.

I would ask to speak to a higher-up. But always keep calm and polite. Look for suitable alternatives, ideally 2 or 3 options. Look up J cabin space on those alternatives (not R space but any J space)

You can say, that you have an even you need to attend at X time and date. The flight was cancelled, not my choice or doing. Here are other *A alternatives that would work with my schedule. There is J cabin space on these flights I have spoken to people who have been accommodated in the past under similar circumstances. I am open to alternatives, but they need to be timely.