If you are reconnecting the Xiaomi Smart Band 9 Pro to a new phone that is still using the same Xiaomi Account, a factory reset is not necessary. The device will remain paired with the account and automatically sync with the new phone once the Mi Fitness APP is installed and logged in.
However, if you are switching to a new phone that is logged into a different Xiaomi Account, a factory reset will be required. This is because the Xiaomi Smart Band 9 Pro is designed to pair with only one Xiaomi Account at a time. To reconnect it to the new account, you will need to perform a factory reset, which will erase any existing data and settings on the band before it can be paired with the new account.