You can't keep band's notification history. But when you sync your band and reset the old one you'll still have all hr, step and sleep data tied to your microsoft account.
Your data is all in the cloud with the Health app. Your data will be messed up the day you swap, though -- the new Band will overwrite the caloric data for that day with your partial day into.
Yea, I swapped mine, wrong size as well. You will lose the old data the day you swap new one. Maybe wait till morning to link. I swapped mine at night not know and lost 10,000 steps from the day Although if you had run data, that will stay....at least mine did.
Swapping first thing in the morning sounds like the best option. I assume the sleep data will be saved. Won't have any step count. Probably only lose calorie count?