My anecdata says once in two decades. I migrated a few simple Perl webapps some time ago, the oldest being 20+ years old or something? Not completely sure when it was created.

I moved them from Oracle to PostgreSQL because we would have had to start paying for Oracle licenses ourselves. It's a very good reason to change databases.

There was no ORM, so I just rewrote the queries to be portable and added some runtime branching in cases where that wasn't possible. PostgreSQL has an extension that adds some Oracle compatibility functions and views; that helped. The initial data migration went pretty nicely with ora2pg.

For the largest (and most important) application I had the writing side duplicate writes to both databases so I could run two instances in "production" and compare their behaviour.

It took a couple weeks of work and testing and then a couple months of observing that the new system works fine; the biggest pain was Oracle's '' = NULL thing and the lack of common SQL syntax for sequences; and the complete lack of tests, of course...