Just read better the title.
Moving from UTF8 wo/BOM to UTF8 w/BOM is not a conversion. All that need to be done is add 3 BOM bytes to the beginning of the file. Just be sure not to add them twice. So basically create a 3 byte file with BOM and do something like (I don’t remember the exact copy syntax of Windows):
copy bom.txt+file.txt file_new.txt
Read this to understand Unicode and encoding. You should realize that UTF8 w/BOM is a pretty stupid concept. So unless you need to feed your file into an application that requires UTF8 BOM stay with UTF8 wo/BOM.