There are tools out there but if you need to move all or most of the categories to tags then it is best to do it manually using SQL.
Remember that category 1 is the default uncategorized so don’t move it. The code below allows you to select which category ids will not be moved over.
the wp_term_taxonomy table manages the relationship between the category and posts. So updating all of the categories to tags will update all posts and what was a category is now a tag.
UPDATE wp_term_taxonomy SET taxonomy = "post_tag"
WHERE term_id not in (1,223,224,225)
and taxonomy = 'category';