Copyright licensing is ONLY assignable in writing.
Copyright is only assignable in writing. The law doesn't require that copyright licenses be formal, written documents. Courts have upheld verbal and even implied licenses. This is a very good thing for open source, actually, since hardly any projects get written licenses from contributors. The mere act of sending a pull request (or sending a patch to a mailing list, or...) is taken as an implied license of the author's contribution, under the license or licenses that the project is using.
Also, good luck getting approval from all 400 - after 20 years some are going to be dead.
That only matters if the heirs object. In this case it's hard to see why they would. The only rational (and I use the word loosely) motivation I can see is a deep-seated dislike of the GPL, since the only real effect of this license change will be to make it completely clear that GPL programs can link OpenSSL.