Ah... I guess the best solution would be to just use SnapMirror, as it can convert from 32 to 64 bit (in 8.1). If you explain your situation to a netapp sales rep, you may be able to get a temporary snapmirror license, just so you can convert the data.
I expect we probably can get a temporary license and potentially add the disks back in after migrating everything. We'll just need to keep the disks in different raid groups since the 32 bit aggrs use 1TB drives and the 64 bit aggrs use 2TB drives :-)
I'm lucky to have some old hardware I can use as a test cluster. I still need to test 8.1 and the conversion process before I'm going to suggest putting it on a production filer. 8.1 is still pretty new :-)
Thanks for all the comments and information.
Jeff