R7800's kernel space was enlarged and the available flash space also otherwise enlarged before 18.06.0 in June 2018. Sysupgrading from earlier builds like 17.01.x will not work. You need TFTP flash for that. That is explained in several places like
But sysupgrading between 18.06 releases, 19.07 build and master builds works quite ok. I have sysupgraded with R7800 to/from 18.06 roughly weekly since then (while testing my R7800 community build).