How to change current category to another in wordpress

I am new to PHP. Please help me to achieve the below scenario. I am trying to achieve this from the last 3 days.

I’ve two categories in my WordPress dashboard named “Available (id:3)” and “Booked (id:4)”. In the “available” category a few numbers are there as posts.

In each post page contact 7 form is embedded.

When I submit a “contact form 7” successfully from that post, I would like to move that current post to the “booked” category.

Please check the link and click any numbers for what I am looking for.

‘Available’ category: http://uei.iwu.mybluehost.me/free212/category/available/
‘Booked’ category:http://uei.iwu.mybluehost.me/free212/category/booked/

I tried something like below in my functions.php and not working at all.