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).
In our case company was strictly "Oracle-only", but one team did a quick implementation in startup-style of Rails and used mysql. No one forced them to migrate to oracle, company just hired DBAs that now mysql.
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).