@Mogbat 

If you have settings in your Deco mesh you don't want to lose (and redo), arrange move from M4 to X20 in the following order:

1. Add X20s to your existent Deco mesh, through Deco app. They will be running side by side with M4s at this point. Use Deco app to verify X20s were added successfully and report as healthy. You might even see some of household devices already connected to X20s at this point.

2. Change Main Deco from M4 to one of X20, see this: How to switch your main Deco to another Deco currently on your mesh network

3. Now that you have X20 as Main Deco, delete all M4s from your Deco mesh, through Deco app. You now have Deco mesh with X20s only. All Deco mesh settings (SSIDs, passwords, parental controls, content fitering) should have been preserved. Your household devices should all have connected to X20s, seamlessly.

@Alexandre. Many thank for your reply So after completing what you suggested I can then swap the main router? 

@Mogbat 

This is correct. It is not recommended to do both Deco hardware and main router swaps simultaneously.

So, swap Deco M4 to X20 while keeping old main router, see that everything works, then swap main router with new Hub 4.

Alternatively, keep Deco M4 as they are now, swap main router with new Hub 4 and verify everything is still working. Then, swap Deco M4 with X20.

@Alexandre. Cheers. Happy New Year