Settings will remain with every upgrade/downgrade, regarding if it is release or snapshot.
But some upgrades will not accept to use the settings, and I dont think you will be able to apply a backup. It is the case with incompatible upgrades, e.g. from old switch to DSA or similar. You will be warned, if that is the case.
I tried upgrading firmware with highlighted file below but it froze. The other file in folder was the original OpenWRT install.
What could of gone wrong? Waited about 5 minutes but my PC with cable connected to router never reconnected. So I hit the reset button to return to original OW image installed.