Changing databases on an existing application doesn't happen, but I've worked across teams within a single company for ~8 years and in that time I've used: 1.

Changing databases on an existing application doesn't happen, but I've worked across teams within a single company for ~8 years and in that time I've used: 1. SQL Server 2. MySQL 3. Neo4j (very briefly) 4. PostgreSQL 5. DynamoDB Each of those was for a different set of applications, and none of those applications changed database, but point being sometimes an engineer will be made to use a variety of databases in their career, even sometimes within the same company (although you could also chalk this up to a particularly laissez-faire style of tech direction).