Question what I should expect here...

I booked a multi-stop trip leaving PRG Dec 15 to Chicago, returning Dec 30 from Seattle to PRG.

Trip was booked on my Merrill+ credit card using points + cash.

I have a PRG-TXL-CDG leg at the start of the journey which is the problem as it's all on AB.

I will be calling Merrill+ Travel (which I believe is operated by Orbitz) but want to understand what I should expect before I do so - is it reasonable to ask to have that first flight changed to a non AB flight? Assuming AB isn't operating those flights in December, what would happen if I do nothing?

Thanks for any guidance!