I believe older libraries should open fine in MEC 7.6.x.  It's easy enough to test it - you can install a trial version of current MEC and try opening the libraries. If you run into problems, feel free to reach out.

Some changes were made several years ago in how task descriptors synchronize with task method elements.

Your old library should continue to work as it did before, but if you want the task descriptors to automatically synchronize with the task method elements, you have to change a preference setting that does a one-time conversion of your library to "autosynchronized". Another change was the addition of "configuration-free" processes, where delivery processes and capability patterns no longer need to specify a configuration.  Existing processes from older libraries will have a configuration assigned.  They will continue to work as before, but you may want to recreate them without a configuration if you want to use the publishing configuration to control what gets published rather than restrict the process to the assigned configuration.

Bottom line - try it out, and if you have issues, reach out.

Cheers,

Bruce MacIsaac