Thanks. That was certainly helpful.
For anyone elses' reference, I was able to upgrade the 4100B via the following process (to avoid the "cannot access package list (.plist) error):
v5.0.1b > v5.1.0d [IBM] (move all files/folders within the v5.1.0d directory to the ftp root)
v5.1.0d > v5.3.2c [EMC] (move all files/folders within the v5.3.2c directory to the ftp root)
v5.3.2c > v6.0.0c [IBM] (move all files/folders within the v6.0.0c directory to a /6.0.0c dir off the ftp root)
v6.0.0c > v6.1.0h [EMC] (move all files/folders within the v6.1.0h directory to a /6.1.0h dir off the ftp root)
Will still need to work on a hardware refresh for this fabric, me thinks, but the immediate issue is resolved.