Software licences neatly fall into three categories - open, closed, and the uncertain - a king of Schrödinger's licence, which must become one of the other two types in time.

Ignorance can only end with knowledge or death. Once someone fixes a mistake in their thinking, by understanding the truth, and how they made that mistake, repeating the same mistake becomes unlikely.

The same principle applies to software licences, and more strongly. Anyone can take a software project under an Apache licence, MIT licence, or anything similar, and change it for another, with very little fuss. But once changed, that licence has nearly no ability to change back, or change to another licence.

Open Licences Stay

Something placed under CC-BY-SA 3.0 cannot later become relicensed under the GPL, unless all contributors agree to make that change. And the various LaTeX packages under the LaTeX Project Public Licence (LPPL) cannot change to a GPL licence.

The near-impossibility of switching one open licence for another means that the choice of licence becomes extremely important. Something distributed under the GPLv3 cannot suddenly find redistribution under the AGPL, or LPPL.

I want to mention licences which are technically not open source, such as CC-BY-SA 3.0¹, or the LPPL², under this section on open licences because people created and used these licences to promote an open workflow. And I want to mention them to note that applying a bad licence to a project often leaves a permanent mistake.

Proprietary Licences Stick

Legally speaking, software projects with a proprietary licence could later become open. For example, Doom received the GPLv2 licence some years after its release.

However, in general, closed-licence projects do not change licences, and with the case of Doom, the project had previously allowed some distribution of its source code for educational purposes, and already had a healthy modding community; this means that it effectively acted as open source software for some time. Doom's relicensing forms a rare exception, and one which could not occur so easily when the software-as-IP has many owners.

No Middle Ground

In time, everything must reach one of the two types of licence, and stay. The MIT and Apache licences only mean the software might obtain a fully libre licence later. So in my head, I always think of them as Schrödinger's licences.