While creating database Data libraries in SMC, Engine is by default and cannot be changed. For example, while creating a new library you select oracle as your database engine would be "oracle" and cannot be changed. Unless you want to just point it to a different schema/db under same database type.
Thanks to pointing it out @Anand_V , didn't realize that it (didn't) work this way.
This is awkward if you want to migrate a whole library to a different database.
Crerating a new Library is easy, but you then need change the Library in each table that uses it as its Physical location.
One idea, you could export all tables for that Libray, and then re-import and map the Libary to your new one? Should at least be less manual work if you have let's say more than 10-20 tables to adjust.