This is definitely a major upgrade. We introduced a complete new platform and architecture starting with jBPM 5. This means that a lot of stuff has changed. There is a migration tool that migrates the old jBPM 3 JPDL definitions into BPMN2 (it's basically an XSLT that converts the XML from one format to the other), but this obviously does not convert any of the jBPM 3 ActionHandlers into the jBPM 6/7 WorkItemHandlers. So depending on the amount of (custom) ActionHandlers you use, this can be substantial.
Second, the APIs have obviously changed a lot as well, so any integration of the process engine with other systems would need to be changed as well.
Apart from these things, also the deployment-model has changed. jBPM 5/6/7 relies on Maven-build KJARs (Knowledge JARs) that contain the process definitions (opposed to the uploading of the JPDL in v3).
So, you can definitely start by using the migration tool to do the initial conversion of your process definitions, but be aware that after that conversion, upgrading the entire system could still require some substantial effort.
Hope this helps,