Unfortunately, at the time of AC 6.5 there were no migration libraries. And as far as I can see, Archicad 24 includes migration libraries for AC13 and later.
So, if I had this situation, I would probably go through those AC6.5 and AC10 libraries, and see which library parts have use for me. Which are the ones that do not have any equivalent in AC24. Then I would take those library parts (and their macros), open them one by one, save them in AC24 format, check whether they work correctly, and if they do, use them.
Otherwise, I would get rid of them and any macros/images they use.
Point is, you should keep your libraries very clean and organized. It can be some work now, but it will pay dividends later in increased productivity.
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