I'm new to the game and wanted to know what happens in this situation.

Jedi Luke with the Yoda Ring gets captured. Rules say that a leader's old ring gets replaced with the captured ring.

But when Jedi Luke gets rescued, does he get his Yoda Ring back since, well, it represents him becoming a more powerful jedi?

Unfortunately no. I understand your thinking though. I would counter by saying that increased tactic values and extra skill icons represent Luke becoming a more powerful Jedi.

Pg. 3 of the Rules Reference talks about attachment rings. It only mentions rings being attached by a card ability, and rings being removed if another ring is gained. If the Yoda ring was meant to be a (semi) permanent addition to Luke, it would be mentioned on the card. I imagine it would say something like "If this leader loses a ring other than the Yoda ring, and has no ring attached, immediately attach the Yoda ring on this leader."

Thanks! I have to admit I'm somewhat disappointed with Jedi Luke and his 3 infiltration and 2 spec ops, making him inferior in missions to, say, Boba Fett with his 3 spec ops or Han + Chewie with their combined 5 spec ops. Ahh, well, just have to adjust I guess. I've only played 3 games so far, probably not enough to have really explored all the possible strategies for winning.

Don't underestimate the power of the dark side triple intel, on either team. Outside of "Lure of the Dark Side", the intel missions are the strongest the empire has. "Homing Beacon", "Intercept Transmission", and "Long-Range Probe" can be devastating to the long term survivability of the rebellion. Not to mention if you want to keep a sizable force at the rebel base space, being able to oppose those missions with Jedi Luke can be invaluable. Pair him with Obi-Wan for that double punch of being able to oppose multiple missions in a round. If you successfully oppose once, the imperial player will have to consider not attempting intel missions, or assigning more leaders to those missions. And leader assignments are much more valuable to the empire.

That being said, Jedi Luke's strength really lies in the Leader Pool. His 3x3 tactics are the best in the game, and 2 spec-ops isn't bad. Plus if you leave him in the leader pool, you can decide where and when to use him, hopefully avoiding his capture and subsequent turn to the dark side.