Advice appreciated on community contributions approach

For example, if you want to relicense a few years later, and can't get ahold of all the original contributors to pay them, that would be problematic. Or if something happened to them you need to find their next of kin.

Maybe it could work in this way: If you relicense, you need to publicly advertise that fact, and the authors then have the option to contact you for $X within 1 year from the announcement.

Not sure how I feel about that approach, though. For one thing, would it be $X per contributor? Then 1,000 lines of patches are the same as 1? Or is it by lines of code? Seems troubling either way.