Just to let you all know, the swap went very well Friday night. We followed the previously mentioned NOW article (Solution ID: 3.0.6869703.2865011) with no problems. The only differences were running a required disk_fw_update after the 840 reboot. Also, the original fibre channel card from the 740 wasn't supported in the 840. Fortunatley the 840 we got came with it's own FC card. However, it wouldn't go in slot 1, like we had on the 740 because it is a longer PCI card. This forced all the disks to be re-numbered, but it came up just fine.
Also, the 840 ran a parity recalculation on all the disks. (Perhaps caused by the disk re-numbering?). This was a bit unexpected, and slightly panicy as all the disk lights were going nuts after we brought the 840 up. But the filer was serving up volumes just fine, so we trusted "The Force" and let it go. It finished a few hours later and all is well. We even added a new tray of disks to get us over 1TB!
Thanks to all who responded.