Understand its perfectly ok for you to leave to follow other interests. That way you're calm and rational.

Then:

Tell them you'll be leaving for this other gig in the fall but are willing to help them in any way until the fall with the cd and gigs.

Kinda like giving 3 months notice at your job.

Downside is they may replace you on the spot; if so, have your ego prepared for it. Really no skin off your nose.

That being said, if this is your career, you may want to have that second bird in the hand before you fly away.

Good luck.