Is it just music or do you have videos, ringtones or apps for iOS?
DRM protected music could upgraded via 'iTunes plus' to files with no DRM…
iTunes Store: iTunes Plus Frequently Asked Questions (FAQ)
There was the option to upgrade at a lower cost at one point in time (sorry it was years ago - I'm insure if this program is still active). This may be more accurate with the current state…
http://www.tuaw.com/2012/12/04/is-itunes-plus-gone-itunes-11-offers-drm-free-aud io-re-download/
Create a Smart playlist where Kind contains "protected". Use that to judge how much you have to upgrade.
The other option (also legal FYI) is to burn Audio CD's of the music & then re-rip it to your Mac. You must use the 'audio CD option' and when re-importing the tags can easily fail to look up in CDDB because the CD is not identical to a purchased one. I would burn as A full albums to give you a hope of tagging the files correctly. This is a painful solution, but it can allow you to play the music without the authentication issues. it also does not hurt to have backups of purchases on other disks - Apple cannot keep all downloads in the store (the rights holders remove them) so iTunes Cloud may not have files you purchased sometime ago.