Trying to bulk replace file types in a group of folders

I am looking to replace a certain file type with another in a group of folders.

I have just over 700 folders each containing a single model of a type that needs to be a different model type. I have done the easy part which is bulk converting all of the models into the new file type, but the problem I am having is replacing the old file types with the new ones efficiently.

Each model is in its own sub-folder in one master folder for all the models (which also includes textures and asset manager data), so the new converted model files NEED to go back in their respective folders.

Does anybody know of a quick way of bulk-replacing all of these files without me having to do over 700 individually?

Thanks

(Image of what I'm talking about)

Assuming you have the new files in folders built in robocopy with copy new files over old ones and do them all simply use robocopy c:\folder d:\folder /mirror it will copy all files including sub folders