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,

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.