Yes, you can replace these Myriad typefaces with the equivalent Myriad Pro OpenType typefaces, some of which were bundled with CS6 and earlier software. The investment in terms of modifying your content is relatively low if you have used styles appropriately and even if not, Illustrator will prompt you if a font is missing and let you globally replace (you would of course, remove those crufty, possibly hacked old Myriad typefaces first). If you don't have access to those previously bundled, the cost of getting a license for them in both $ and time is quite low compared to other alternatives.