Mandatory attribute to be filled in every state.

We need to create a field/attribute in our work item which will have the following properties:
Similar to Actions/Comments field in CQ.
If one updates the field and hit save.. the data is swapped to another field - say <Comments>
When a new user tries to change state (or same user tries to) the Comments field appears empty. Since the data has been swapped/archived to the log.
This is different from the History function of RTC because history does not remove data from existing fields, just takes a snapshot.

Clearly : Say example if we create a wi with an attribute "comment" mandatory. and it should be filled in every state before moving to the next.
It is working well when we try to create wi but it is letting it to move the state before filling/updating the "comment" mandatory field.

Thanks inadvance.