How do I replace my old Charge 4 with a new one on Android 8.1

[teethiob]

MY phone is on Android 8.1 (last physical keyboard phone manufactured). No updates for Android or Fitbit App which is version 3.75.

Lately the battery life of my Charge 4 decreased to 1,5 days. So I decided to buy a new Charge 4, it is on it's way now but before that I bought also Charge 6. When I tried to add Charge 6 to my Fitbit account, it says I have to move my Fitbit account to Google account. According to help, for moving process I need the Fitbit app updated to latest version, which is not possible because I can't get any new updates on my Android 8.1 phone.

So no luck with Charge 6. Now I have a question, is it even possible to replace my current battery dead Charge4 to a new Charge 4 without updating Fitbit app, moving Fitbit account to Google or upgrade my phone to newer Android version? During the adding new Charge 4 process I might end up in a situation where the old Charge 4 will be deleted from my account and it is impossible to add new Charge 4. 

Question: is it possible to replace Charge 4 on my Fitbit account when app version is 3.74 and Android version is 8.1 and phone's Android version is 8.1? 

[LZeeW]

@teethiob  To the best of my knowledge, your phone must run Android 10 or higher to replace a device.  You are setting up a new device, which just happens to be the same as the old device.

BTW, Google ended all security support for Android 8.1 in 2021.  Please keep this in mind 

[teethiob]

Answering to myself. Bought a brand new Charge4 and managed to replace my current weak battery Charge4 on Android 8.1 phone with Fitbit account. Tried to do this with Charge6 and prooved it is not possible.