>If I create a UserControl in solution A, then reference it in solution B, then go back to solution A and make a change, how do I ensure that the change makes it to solution B?

I assume you mean:
>If I create a UserControl in a project in solution A, then reference that project in solution B, then go back to solution A and make a change, how do I ensure that the change makes it to solution B?

If so then the answer is that it should be automatic when the solution is rebuilt. If it's not what you meant please expand...
Best,
Viv