Apology in advance if I'm telling you something you already know. Perhaps it will help others in similar situations.
If you have a licensed copy of Illustrator CS5, that would be a perpetual license. You can just open your legancy files with it, and save them as either AI files with the PDF compatibility option, or as PDF with the Maintain Illustrator Editability option. Either way, you will then have files that contain two complete versions of the content: the "dumbed down" constructs of PDF, and the fully-editable constructs of the native Illustrator objects.
For example, if your Illustrator file contains a Blend, that construct will be just a stack of individual paths in the PDF version which other programs can open, not a live blend with adjustable parameters. There are many other similar object-specific situations.
The point is, it's best to retain the fully-editable native constructs if you can, even if (like me) you never intend to rent software from Adobe. You never know when you may need to go back and edit the files in their native form.
If you are saying that you replaced a dead Windows machine with a Mac, and no longer have access to a Windows machine, you should be able to run your existing copy of Illustrator CS5 with a Windows emulation (Parallels?).
(Frankly, if I were still working on MacOS, I'd at least still have a current Windows machine around, if just for practical workaday compatibility with Windows-based clients.)
You can automate opening your Illustrator files and re-saving them with PDF compatibility (or saving them as PDFs with Illustrator editability) by using an Action (Illustrator macro) in Batch Mode. Batch Mode lets you run the Action on a whole folder full of files at once, making it practical to re-save hundreds of files at a time.
(User-defined automation by macros and Javascript scripting are major features which I hope are at least planned for the Affinity applications.)
