Many tag programs do that, but I think most of them are crap. By "easy" you probably mean a Windoze GUI program, so I recommend you
The GodFather. I'm not sure if it's the best but it seems to me that it's one of the most featureful tagging programs.
You enter the relevant directory, enter the "Tag" mode, scan the directory if it's not automatic, and use the "Sync" button which can copy or move ID3v1 -->ID3v2 or vice versa. It can even limit the usage of v2 to where it's needed for field types or lengths.
Many people here dislike ID3v2, but you've probably heard about it and decided on it. If not, search the forums.