A fairly large number of projects decide to change their license later in life, but this is mostly if they start off with more "restrictive" (for lack of a better term, i know how Free software advocates feel about this) licenses, like GPLv3 or whatever, or because they discover the license doesn't work well for what they wanted.
For example, Eigen, a very widely used math library, switched licenses because they were mistaken about the implications of LGPL, and as it started getting more widely used, it started affecting usage.
Other projects grow runtime libraries, and discover they don't want licenses that require attribution for those, because then everyone who makes a binary has to ship notices, etc.
There are even simpler cases.
For example, llgo, the Go language frontend for LLVM, is being contributed to the LLVM project. This requires a license change to the same license as LLVM. Because there were no contributor agreements, every contributor had to be tracked down and asked. A bunch are either dead or not around anymore, and now those contributions have to be rewritten or excised. This has slowed them down a few months so far (at least from my view, pcc can surely correct me if it hasn't been that long)
In terms of how often this happens, i've personally helped about 50 medium-high profile open source projects change licenses, and it's not even my "real job".
It's fairly common that as a project goes from nothing to having a lot of users, they end up having to change something about their licensing.
For example, Eigen, a very widely used math library, switched licenses because they were mistaken about the implications of LGPL, and as it started getting more widely used, it started affecting usage.
Other projects grow runtime libraries, and discover they don't want licenses that require attribution for those, because then everyone who makes a binary has to ship notices, etc.
There are even simpler cases.
For example, llgo, the Go language frontend for LLVM, is being contributed to the LLVM project. This requires a license change to the same license as LLVM. Because there were no contributor agreements, every contributor had to be tracked down and asked. A bunch are either dead or not around anymore, and now those contributions have to be rewritten or excised. This has slowed them down a few months so far (at least from my view, pcc can surely correct me if it hasn't been that long)
In terms of how often this happens, i've personally helped about 50 medium-high profile open source projects change licenses, and it's not even my "real job". It's fairly common that as a project goes from nothing to having a lot of users, they end up having to change something about their licensing.
It's also a massive pain when there are no CLA's.