Finding all previous contributors might be difficult, but the hardest part of a relicensing effort is that a previous contributor may have died. At that point, the copyrights they had are passed via their will (usually to a spouse or child) and now you have to convince someone who might not even be aware of Free Software as a concept to make a decision about their departed relative's views on a topic they knew nothing about.
I think if you gave people the option of signing something that allows relicensing of their code under copyleft agreements without needing explicit permission, a lot of people would take it. Tracking folks down who have been gone from your community for years takes a lot of effort.