There are a couple of ways to handle this. First you could create an automation like this: http://drop.userscape.com/Wr2o/1baxXHOy. The one downside to this method is that all the request would be assigned to the inbox as well as having their category reassigned. You could also choose to assign the requests to a specific individual.
A second option would be to use a database query to make the adjustment. Something like
update HS_Request set xCategory=1 where xCategory=2
The disadvantages of this approach would be that this change won't be logged in any way and if you have staff assigned to particular categories you could end up in a situation with a request assigned to an individual who doesn't have rights to that category.
Finally, you could also use the web api request.update method to make this mass change (https://support.helpspot.com/index.php?pg=kb.page&id=164#private.request.update)