I have a client who is on a unix server using a very old version of Mas90 (I believe 3.xx)
What's the best way to migrate to a windows OS using Sage 100 ERP 2015?
Is there a utility that I can use which will convert those files?
You don't need to convert files stored on a Unix file system in order to use them on a windows file system. You just need to figure out how to transfer files between the two systems. This could be easy or very difficult depending on how they are configured.
If your Unix system is able to read some Windows file system, such as FAT, then transferring the files may be as easy as hooking up an external drive (or temporarily install a drive in the server if USB is not available) formatted with this file system, and copying over the files. FAT compatibility is common with Linux servers, but not Unix.
If the two systems are unable to interact with each others file systems, then my next thought would be connecting the two file systems over a network. Options that will work for you depend on what is installed on your Unix system and whether you are able to install additional programs. More importantly, it depends on your level of experience with Unix (or if you're able to get help). Some options that come to mind are:
1) FTP or SFTP (given there is FTP (or SSH FTP) server software installed)
2) NFS (Network File System) or Samba
3) Use secure network copy command: pscp
If you aren't sure how to determine if you have FTP or SFTP servers installed, try connecting to the server with a free FTP utility such as FileZilla: https://filezilla-project.org/
If it connects, then, you're golden.
*Community Hub is the new name for Sage City