I *think* once there's glandular development, it doesn't go away (if it did, trans men wouldn't need top surgery). Fat redistribution might change with a significant enough period of time.
I went off E for 3 weeks pre-surgery, and didn't notice any real change in actual development, but my boobs did seem less perky somehow. I think it's the growth cycle; mine seemed to get perky for a bit, then soften and flatten out as fat was layered on, then repeat. So I'm purely guessing, but taking away E during a "perky" phase might make them stop growing and look a bit softer and droopier.
No long-term effects, thank goodness. I am even still growing at 2 years post-op.
