You will need to ensure the 3G MSC node has the right classmark RFC values are configured.
I worked on a project that involved handover between Huawei 3G and Ericsson 2G. I am pretty sure the procedure is the same, irrespective of the switch.
Make sure the 3G node can receive the classmark values sent by the 2G MSC.
Usually, the problem will be from ciphering and classmark parameters. Make sure the 2G MSC node has the same config parameters as the 3G node.
The best way though, is to check out your support site for similar project experience that has been shared (I am assuming you have acess to Alcatel-Lucent site)
I hope this helps give you an idea of what to look out for.