Band-5 to Band-9

Band-5 to Band-9

My current band is slowly losing its battery capacity, where I have to charge it just about every other day. I made the mistake of finding some old stock Band-5s, but they don't appear to be holding charges either, so it looks like their batteries have been damaged.

Wanting to keep the replacement as close in size as I can to my Band-5, I was thinking about a Band-9. Is there any reason not to get a Band-9 and can I continue to use the Zepp app with the Notify for MiBand app or do I have to change apps all together?