So, what the issue turns out to be is that the factory preset that my user preset was based on a ModFactor preset. As it turns out, what that seems to mean is that while only FXA is indicated to be in use, the algorithm contained, because of its size, crosses over into the memory alloted for FXB. So, when trying to store an additional algorithm in FXB, the interface just blanks it out, effectively disallowing a transfer to that block.
Since the info alluding to limitations in the use of these presets is in the preamble of the ModFactor presets, it may be useful to make a note of how the "limitations" manifest themselves in the preset editing interface.
