I would see a potential increase in the level of modal remoteness:
If (in the future) a tariff
- is levied (open, not remote)
- was levied
- were levied
- were to be levied
I would say
? was to be levied
in not generally acceptable, possibly because the form with "... to be" is the most remote, which conflicts with was (as less remote than were).
However, there is a lot of overlap, and it is complicated by
(1) some sociolects that avoid "were" and some that avoid "was"
(2) stigmatization of the modally remote was, and possible hypercorrection.
The complications can make it difficult, for instance, to work out what was really said in the case of indirect reported speech (as I think we once discussed).