16.04 is beyond end of life.
You’ll need to resize your disk. You cannot resize to a smaller disk, so once you go bigger you cannot go smaller. You probably want to switch to a newer cpu anyway.
If you don’t care about down time, the upgrade will likely work fine.
If you create a new droplet you can switch with only read only time (if you copy the ssl certs over) and if something goes wrong, you just keep using the old one.