Based on my understanding : The custom jars are not re-built for DevTest 9.1 environment. They were copied straight from CA LISA 7.5 and are being used in DevTest 9.1.
Please note that CA LISA 7.5 and DevTest8 were supported on Java 7. However, DevTest 9.1 is supported on Java 8.
If you have jars of unsupported version in your DevTest, then it will be considered as unsupported environment. Hence, try to fix that as the first step, before you proceed further.