So, if I want to upgrade from Octopi .15 to .18, I basically need to create a new SD card with the new image and then restore my saved settings. Am I missing anything?
Will both versions continue to be supported?
No.
No. I fade out environment support for older OctoPi images gradually, and it's time to give the almost three years old 0.15 the axe, probably with 1.6.0+. Every supported base image means at least one additional upgrade test for each OctoPrint Release and Release Candidate, and endless headaches due to an outdated basic environment on top of that.
I didn't realise that.
I have now resolved this by editing the bashrc file with command:
nano ~/.bashrc
I scrolled down to the bottom of this file and added the following command to the file:
alias python='/usr/bin/python3'
My Python version is now Python 3.7.3
Hello there. So in the post it says the main reason to update from 0.17 to 0.18 is Python 3. If I already updated my 0.17 to Python 3 when I upgraded OctoPrint is there still a compelling reason to re-flash? I am considering doing it anyway, but I did make some custom changes to add ATX-Pi and change bluetooth settings I would rather not have to redo unless I have to. 
Thanks for all the work,
In the short-term, if you are happy with OctoPi 0.17 running OctoPrint 1.5.3 under Python 3 on your current hardware then there probably isn't a compelling reason to re-flash. The new version of OctoPi is required to support newer hardware (i.e. RPi 4 latest revisions) and includes the latest (or almost the latest) version of OctoPrint configured to use Python 3.