Giving my tests more coverage and thinking about reasonability of an SRU:
Vivid is soon out of scope and has already 0.98.7* packaging due to security updates - so V->W path isn't important anymore.
Also the Trusty -> Wily path can be considered unimportant now that Xenial is out a while (and almost nobody does LTS->nonLTS).
That said I'd vote to only fix T->X.
Giving my tests more coverage and thinking about reasonability of an SRU:
Vivid is soon out of scope and has already 0.98.7* packaging due to security updates - so V->W path isn't important anymore.
Also the Trusty -> Wily path can be considered unimportant now that Xenial is out a while (and almost nobody does LTS->nonLTS).
That said I'd vote to only fix T->X.