How to assume packaging downgrade in hard case ?

I'm packaging Bugzilla but at now I don't know how to make downgrade possible. After installing the last package version (not public at now), it will not be possible to downgrade while there is major change in the data structure. I've try to resolve some by making the prerm script change back the

How to assume packaging downgrade in hard case ?

I'm packaging Bugzilla but at now I don't know how to make downgrade possible. After installing the last package version (not public at now), it will not be possible to downgrade while there is major change in the data structure. I've try to resolve some by making the prerm script change back the