Additionally : iodéOS system upgrades generally do not need such kind of manipulation. This is due to a change in the LineageOS updater blocking transitions of major updates until a system property is set, that we saw too late.
And by the way, another solution exists, instead of installing the v6 to v7 updater : setting that property to true, by issuing the command adb shell setprop lineage.updater.allow_major_upgrades true.