From the cco i have found that its not possible to migrate a callmanager 4.2.1 with a data migration tool to callmanager 5.1.2b. Can any ony confirm this.
Sadly, you are correct :( There is no Upgrade path from the CCM 4.2 Train to any 5.x Release. This nice new Tech Note document shows a good visual of the supported Upgrade paths (CCM 4.2 can go directly to CCM 6.x)