joarc, I think it's really nice of you to even wonder if you're asking too much of your travel agent. I'm sure her first consideration is to do the best job she can for you. The changes you've made happen all the time, and are part of her job. If your bookings qualify for the lower fare, I'm sure she will be happy to make the change for you.
A gift certificate certainly is very nice thing to do, and I'm sure your agent will be most appreciative.