"Once you have released you Kuriboh, you will be unable to obtain Rewards for forms beyond the form you released it at."
Correct me if I'm wrong, does this means if I release it at Sphere Kuriboh for the 1st time, then I can still raise another one, but at max it can be raised to Sphere only.
That's probably what it means. But I guess it's possible that it could just mean that you forfeit any further progress on that Kuriboh (which is blatantly obvious, but people are stupid).