Copying a workflow from one list to another is a pain even in the same site. If you have lots of steps and all you want to do is change fields in each of them then it might be quicker to do this than start from scratch.

I just opened the XOML in notepad and copied it’s contents into the XOML file of the new workflow (see step 3 in link). Then I copied across the *file*.xoml.wfconfig.xml and *file*.xoml.rules files and then went in an edited the workflow normally in SPD. Bit of a pain so if you don’t have a complicated workflow it might be easier to start from scratch.