I use a command line tool called
zipconv, which is a windows batch file. It does require
7Zip so do install that before using it. (You probably should have 7Zip installed anyway - it's very good.)
I just put the batch file in the same directory as the comics, open up a command window and enter the following commands (without quotes).
"ren *.cbr *.rar" [enter] - this renames all the CBRs to RAR files.
"zipconv *.rar* [enter] - this converts all the RAR files to ZIP files.
*ren *.zip *.cbz" [enter] - this converts all the ZIP files to CBZ.
It's all basic Windows command line stuff - it works because CBR and CBZ are just standard archives with the extension changed to make them easier for comic reader programs to recognize.