HappyFox lets you change ticket categories automatically via Smart Rules, upon satisfying a condition.
Available in all Pricing Plans
Go to Automate >> Smart Rules.
Click on Create a Smart Rule.
In the actions section, under "Set Ticket Properties", choose "Set Category".
Choose the destination Category from the drop-down.
Define conditions that govern when the Smart Rule should be triggered automatically.
Enter name and description for the Smart Rule. Choose the categories you wish to move tickets from, enable "Execute smart rule only once when conditions match" if needed and associate an appropriate Work Schedule.
Finish Creating a Smart Rule.
To know how to set Category as Action in Smart Rules in HappyFox Classic, refer to the document attached.