I would give a try to policy sharing feature in this respect. This could result in the same policy groups appearing on the ePO 4.6 relevant policy categories with the owner as the ePO 4.5 server. Once the policy is shared and have appeared on the remote server, you can duplicate it to make it local and stop sharing.
As prerequisite you need to register the ePo 4.6 server onto the ePO 4.5 server.
If there should be any compatibility issue between 4.5 and 4.6 this will be shown at this very first step. Consult the ePO manual for policy sharing steps (there are 3).
Policy migration across major versions by hand would not be recommended due to possible schema differences.
What you could consider however is building your new server to be ePO 4.5 and the same patch level as the old server, then you can export and import policies from old to new.
Note that when you import a policy to a new ePO server it goes into the policy catalog, you will still need to apply the policies to the respective groups in your system tree.
When happy with the result, upgrade the new server to ePO 4.6 and the ePO 4.6 installer will migrate the local policies up as needed.