During version changes of openwrt there were changes in naming the images, too, for WE826.
You can do "sysupgrade -F -n -v imagename"
to bypass the name checking.
I remember to have done this myself some version(s) in the past on my 826.
( I also remember my swearing about this 'enhancement' ...)
Using emergency boot of the 826 and browser, you can flash back 17.01, in worst case.