[justinclift]

The standard way of "relicensing" a project is to contact all of the prior code contributors about it and get their ok.

Generally relicensing is done in good faith for a good reason, so pretty much everyone ok's it.

Trickiness can turn up when code contributors aren't contactable (ie dead, missing, etc), and I'm unsure of the legally sound approach to that.

[toyg]

The legally-sound approach is to keep track of your actions, so you can later prove you've made "reasonable" efforts to contact them.

[Meneth]

If a copyright holder does not give you permission, you can't legally relicense. Even if they're dead.

If they're dead and their estate doesn't care, you might pirate it without getting sued, but any recipient of the new work would be just as liable as you are, and they'd know that, so I probably wouldn't risk it.