You may have experienced that some of the web apps refuse to accept any type of archives other than the most common ones like ZIP or RAR. In these situations, you have to extract the contents of your non-standard archives and then repack them into ZIP or other supported formats. Instead of manually changing the archive formats, you can use ArcConvert. It is a free software that lets you convert one archive format into another without having to handle extraction of files from old archive and then packing them into new archive format yourself.
ArcConvert can convert archive to and from dozens of archive formats like ZIP, RAR, 7Z, CAB, ARJ, BZ, UHARC, TAR, LHA and many more. It does not support RAR format natively and you must have WinRAR installed on your system to support the RAR format. This is perhaps due to copyright issues, but you can install WinRAR in a few seconds.
The ArcConvert GUI offers you to select one source archive by clicking on Open Archive button and then choosing the file. Then you have to select a target archive format from the drop down list box. And finally you have to click on the Convert button to proceed with the conversion.
Since the archive conversion is attempted using the command line tools that come packed with ArchConvert, you may see a command line interface window (CLI) displaying the whole progress of conversion. Some of these tools are very old but they still work. For example the UHARC command line tool was written in 2001 but manages to work in Windows 8.1.
After the archive format has been converted, you are notified to the effect along with an option to remove the old source archive if you do not need it anymore. It is recommended that you keep the old archive for backup.
Verdict: ArcConvert has very stale interface and makes use of decades old command line tools, but still manages to do the job of converting one archive format into another. Though it supports a large number of archive formats, it may not support newer archive standards introduced in the recent years.
You can download ArcConvert from http://archivconvert.sourceforge.net/.