I have a 16gb eMMC for my ODroid N2. HA says it’s already at 66% usage, and I was just thinking of adding longterm sensor data which will make that much worse. It’s also been less nine months and HA says it has only 80% life remaining.
What size should I upgrade to? Is 128gb overkill?
Once I get the new one, how can I take my currently working install and migrate all the contents to it?
You are lucky!
That eMMC degrade fast when permanently writing on them (DB 
) is expected (write amplification 
). Mostly they consists of the cheapest flash cells (the same you typically find in sd cards and usb sticks) and are typically the opposite of durable.
Sounds good. There is essentially no overkill - only over-provisioning which helps to strech the lifetime of your flash storage by having lot’s of extra fresh cells at hand 
From Wikipedia, the free encyclopedia
Overprovisioning can refer to:
- Allocation of additional storage space required to mitigate write amplification in a solid-state drive
Backup 
Restore 