No. 1 Sentai Gozyuger Episode 29- "The Ultimate Hospitality! Ryugi Is Serious!"
 

Ryugi, worried about TegaSword, makes a serious decision...

So according to Rei, the Ring that he possesses is actually from the Calamity. Looks like the Calamity's return is imminent even with Pestis (from the summer moive) defeated.

And Ryugi being replaced? Seeing how Mashiro's flashback revealed there's a Gozyu Leon predating Rikuo, would this make his replacement the second or third Gozyu Tyranno?

I'm wondering about this as well, a ranger getting replaced - especially more than halfway through the season is extremely unusual for Sentai if this is a long-term or permanent change

I wonder if something happened behind the scenes on the production side of things or if this was always the plan?

This is most likely a temporary thing and is probably a homage of the original sentai series Gorenger, where the Yellow ranger was temporary replaced until the replacement ranger was killed.

I've been thinking, it would be insane if this somehow doubled as a Lost Galaxy reference. Different color, sure, but still.