WinRAR used many naming sequences for split multi-volume archives through its numerous versions. You cannot convert individual pieces between them. The only thing you can do is merge the archive and extract it. As a whole, this conversion does not make much sense.
Convert RAR compressed files from a multi-volume archive to RAR compressed files from a multi-volume archive.
We found one software available in the database for this conversion.
Microsoft Windows software - convert r59 to r77 on Windows
One of the oldest and most popular shareware file archiver and data compression utility
Convert from r59 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.