It’s a lot more than title. Passing game coordinator helps design the weekly passing plan. Bigger responsibility and increased pay as PGC than a WRs coach. PGC, like QB coach, is also a pathway to become OC.
I think if Curry was offered to continue being QB coach, he may have stayed. He’d still have a significant position on the staff as the most important position coach. Going from PGC/QB coach to just WRs coach, his first position coach job on the staff, is a big step down if Curry has goals to keep climbing as a coach.