30 years experience running enterprise development teams here. Switching databases has happened once for me, Sql Server to Postgres. We were busy with a huge rewrite of something existing, approx $100 million project for a major company.
The instruction to switch dbs came midway through the project, basically on the whim of the CIO. Luckily we were only lightly impacted by db specific features on a couple procs, but code base was abstracted away - which made it achievable.
I thought it was hilarious