we may need downgrade preferences for the upgrade. e.g. we have qca in xenial but don't need one in bionic currently, so Jon made an apt preference to push bionic to the ubuntu qca. since this only applies on bionic though what happens is:

• upgrade
• still xenial qca
• user dist-upgrades at some point in time
• now has bionic qca

this is a major binary compatibility concern and needs to be different.

there are facilities in the upgrader to apply "quirks" hacks to the system. on part of this is live patching, that seems not useful because it appears to apply to pre-existing files (which we don't have, could do if we wanted but seems a bit complicated). another thing seems to be some sort of hooks mechanic PostInitialUpdate and PostCleanup which would allow us to run AFAIU arbitrary code at various stages of the upgrade. we could write a preference there. lastly we could just dump in new code to hook where we want the way we want.

in the end we'll definitely need to install all the preferences we want before actually calculating the update though.
ideally this would be in files with a special DIFFERENT name from the regular preferences in bionic. e.g. 98-neon-upgrader-preferences. this file should then be ripped out after the upgrade is done. in addition, regular preferences will be installed by neon-settings, these are only useful for already existing preview installations though. i.e. after the upgrade all of the preferences should already have been applied!

(that got a bit rambley; apologies)