There may come a day when you decide that either you want to convert your categories to tags or vice versa.
Suppose, for example, you’ve found that you keep reusing a certain tag again and again. After a certain number, it occurs to you that it might be worthwhile to create a category for that term.
Of course you could manage to do this manually, but there’s a much easier and automatic way – with a plugin called Taxonomy Converter.
Using Taxonomy Converter