In bug 1607798, a watershed was added to not skip 72.0.2, to ensure that installations would migrate off of an old database before support for it was removed in 73. This assumes that an update can only be downloaded from a running Firefox instance, so 72.0.2 will run at least once in order to do the migration. Without specific support, the update agent will invalidate this assumption.

This is probably a sufficiently rare situation that it may not be necessary to add general-purpose support.

One alternate solution would be to add a flag to the update URL that records whether the migration has taken place; if the flag is missing or not set then Balrog can refuse to advertise an update past the watershed. This would lose some of the benefit of the update agent, but again if it is rare this may be enough.