Mozilla relicensed code from 445 contributors (from MPL-only to MPL/GPL/LGPL tri-license). It took them 5 years [1]. Consider complications like finding the relatives of a dead contributor, getting them to listen to you, then to understand the issue, then to agree. The Linux codebase is much older and has accumulated more contributors over that time, so the task is much larger, too.

1: http://blog.gerv.net/2006/03/relicensing_complete/