Is it doable to upgrade Cisco Call Manager 4.2 to Cisco Unified Communication Manager 7.1 ?
Yes, 4.2(3) would be the minimum required version
DMA for Cisco Unified Communications Manager Release 4.x
DMA release Notes
Data Migration Assistant (DMA) Release 7.1(3) collects application data from Unified CM Releases 4.1(3), 4.2(3), and 4.3(2) system for upgrade to Cisco Unified Communications Manager Release 7.1(3). DMA exports data in the current Window based system (with SQL server database) that later gets imported to the appliance-based system (with Informix database).