^^^
It is my understanding that it is an "engaged to be engaged"" sort of a thing. My DH gave my a promise ring when we were in high school - too young to be engaged (for us) but we were very committed to each other.
Thank you Greenpixie!
I agree with the others that it sounds like you're looking to upgrade the promise ring to an engagement ring. A 1.5 carat solitaire worn on the wedding ring finger is definitely an engagement ring, no matter how you slice it. I also agree that it would probably seem ungrateful to your boyfriend to ask for an upgrade in the current situation and especially since he didn't give you the promise ring so long ago. It sounds like the ring he gave you is appropriate for its purpose as a promise ring. What you're looking to upgrade to isn't appropriate, it's an engagement ring.
The disconnect might be that your boyfriend is a little old for a promise ring arrangement, while you're probably not too old for it, but close. Perhaps you really *are* looking to be engaged?