My approach for replacing current Ubuntu with newer
First of all, do backups. They are never extras. Then check twice and thrice what you are doing. Verify that the partitions you are removing are the ones you want to remove, by the way, is not necessary that you remove the partitions, just format them.
If you want, you can reuse the old /home just unticking the format option in the Something Else dialog. That will save you a step. Don't use btrfs for /boot, but ext4. BTRFS is still experimental and may have problems if you use it for GRUB.