There are two stages, neither terribly complicated (three if you count research).
For each clothing object, open it in the CS, and find what body parts are used.
For each body part, open it, change to armor, change the ID, and save.
Then create a new armor object with the new body parts.
The will create armor from the clothing.
Now, if you want to just replace it, things get somewhat more complicated. You'll need a hex editor or other file editor and replace IDs, and it may crash the game. It's much simpler and safer to make clones as the new type and use them when/where necessary. If you have to do a replacer, we may be able to figure something out.