There's a lot of bowlers on here that can tell you more on this subject .. but my experience:
I was having leg problems .. buddies were suggesting 16 pound ball are obsolete and I should drop to 15. Started buying 15's .. maybe accumulated 4 balls. Found I was throwing a little higher speed and was not getting expected reaction. Took a while and lost some average but I finally got it to where I was happy.
Loss of average could have been my leg but I think it was more not getting the reaction expected.
YES I have gone back to 16's and I'm feeling comfortable. I do happen to be a Senior bowler!

The ball is lighter; so I found several differences.
1. The RG, Differentials and Intermediate Differentials are different. Some are large and some are small. So many of them have to be drilled differently to get a similar reaction.
2. You will throw it faster, in general. This means you not only need different drillings, as in #1 above, you also may need slightly more surface, when bowling on the same oil conditions.
3. I found (you may not) a little more difficulty in getting a clean release because I like a tight thumb hole. 16s not only have more weight, they have more momentum during the arm swing. So they pull the ball off my thumb more easily. You may or may not have this problem.
4. Because of the lighter weight, you may find yourself with more revs than you had before. This affects overall hook, breakpoint, but can be controlled with different drillings and surface, as mentioned above.
5. #4 affects carry power; if you don't control some results of the lighter ball, you may leave pins that you didn't leave with 16 lb/ balls. Power and drive and hitting power are not lessened with 15 or even lighter balls, if you take some of the above factors into consideration.

I'd suggest testING a 15 lb., using the same model as one of your favorite 16s. Make sure the driller knows what you're doing and the difference in RG and Differential, going from 16 to 15, BEFORE DRILLING IT!!