According to our official documentation (Bamboo Upgrade Guide), Bamboo's upgrade should work as:

When upgrading from Bamboo 4.0 and later, you can upgrade directly to the latest version of Bamboo:
4.0+ >> LATEST

However, according to Upgrade task 4408 fails with: "java.sql.SQLException: ORA-22859: invalid modification of columns" when upgrading Oracle's database (from 11g to 12c), an intermediate Bamboo version (e.g. v5.11) is required.