ITCZ,
As Bill has provided with an example of how Vat may change, i.e. a new configuration other than the normal, flapless, partial flap, partial or full anti ice/deice or in icing conditions, failure of one or more AoA meters, Air Data Computer failure and failure of ....it goes on, this can even be without an engine failure.
Some aircraft also require handling speeds above normal speeds in icing conditions.
The most extreme example I can think of in GA applies to the PC12 which can have a Vat in icing conditions of 102 kts (CAT A->CAT B), or 134 kts for failure of the pneumatic deice boots in icing conditions, flaps 0 (i.e. change from CAT A -> CAT C).
Some aircraft operators require a handling speed when circling above 135 kts single engine, even when CAT B two engine, SF340 comes to mind.
Bill has kindly provided a ref for you already for the increase of CAT (JEPP TERM 2.1.2 (B)), and note CASA may permit an operator to reduce its CAT, (JEPP TERM 2.1.3 refers).
