Mass Shifting the 5000 posts to new category

Backup your database.
Then make sure you backed up your database!

Delete the Category that contains those posts and you can assign those posts to a different Category.

Or, how about just renaming the old Category? (only works if ‘new’ category does not exist).

Resources:
WordPress Backups

Thanks Michael,
It was quick. But solution is not so simple. I have a membership blog with plugin wpb-paypal and posts are in ‘uncategorised’ category. Which has been assigned as private. Making ‘uncategorised’ private made my home page(static) invisible to guests. I have realised it now that making this category public can make my home page visible to guests. Therefore I need to transfer all my posts in Uncategorised to some other category. Pls. provide a solution.
Yogendra Singh

Hey Miky,
where are you man? I don’t see anyway to PM you man
Yogendra Singh

1. Backup your database.
2. Create your new category.
3. Change your default Category to new category.
4. Delete ‘uncategoried’ category and assign to new category.

Am I missing something?

There’s also Batch Category plugins but don’t know if they work at 2.3.1.

Hi Mike,
I am using 2.2.1 not 2.3.1. Which are these plugins. Deleting uncategorized lives my home page hidden. My page is shown to guests only when uncategorized is set to public. Thats why I need no posts in this category.
Yogendra Singh