Transitioning from a software engineer to a PM is much easier than the other way around. In fact, the other way around is rarely done, because the requisite skills to move into software development are arguably harder to pick up on the job, whereas good PM skills aren't difficult to learn along the way, especially for someone who's already interested.

I can only speak for larger companies, like Microsoft and Facebook, but I'd say in larger companies that there are a few easy times in a career to transition, and a few hard times. In general, early in career is an easy time to transfer. This is because most entry-level people are primarily learning their profession anyway, and so if you were to switch, you'd be treated like everyone else who's new to PM. Once you get to a middle "journeyman" level (like L63-64 at Microsoft, or E5 at Facebook), it actually can become harder to switch. This is because your peers at that level are all quite good at the PM skillset, and you'd be compared with them when judged for performance. Not impossible to do, but harder. Funny enough, later in your career, at the Principal or Partner level of these firms, things can once again get easier (to switch from dev to PM); the reason there is that at those levels, many of the skills between those functions are once again shared. People at those levels tend to spend much of their day communicating, leading, and planning. Thus engineering and PM jobs can actually look pretty similar later in career.

I encourage people to switch within the first few years of their career, or perhaps 10-15 years in. It's between years 3-10 that it's probably harder. Not impossible, but harder.

I hope that helps!