Unable to update using update site mechanism

[sjiherzig]

I've been unable to update an existing OML Workbench product with new releases using the software update site mechanism. For an update from M20 to M21, the following error message is thrown:

Your original request has been modified.
  "JPL's Ontological Modeling Language Core Features" is already installed, so an update will be performed instead.
  "JPL's Ontological Modeling Language UI Features" is already installed, so an update will be performed instead.
Cannot complete the install because of a conflicting dependency.
  Software being installed: JPL's Ontological Modeling Language UI Features 0.9.0.0-M21 (gov.nasa.jpl.imce.oml.ui.feature.feature.group 0.9.0.0-M21)
  Software currently installed: OML Workbench 0.9.0.0-M20 (gov.nasa.jpl.imce.oml.rcp.product 0.9.0.0-M20)
  Only one of the following can be installed at once: 
    gov.nasa.jpl.imce.oml.viewpoint 0.9.0.0-M21 (gov.nasa.jpl.imce.oml.viewpoint 0.9.0.0-M21)
    gov.nasa.jpl.imce.oml.viewpoint 0.9.0.0-M20 (gov.nasa.jpl.imce.oml.viewpoint 0.9.0.0-M20)
  Cannot satisfy dependency:
    From: OML Workbench 0.9.0.0-M20 (gov.nasa.jpl.imce.oml.rcp.product 0.9.0.0-M20)
    To: gov.nasa.jpl.imce.oml.ui.feature.feature.group [0.9.0.0-M20]
  Cannot satisfy dependency:
    From: JPL's Ontological Modeling Language UI Features 0.9.0.0-M20 (gov.nasa.jpl.imce.oml.ui.feature.feature.group 0.9.0.0-M20)
    To: gov.nasa.jpl.imce.oml.viewpoint [0.9.0.0-M20]
  Cannot satisfy dependency:
    From: JPL's Ontological Modeling Language UI Features 0.9.0.0-M21 (gov.nasa.jpl.imce.oml.ui.feature.feature.group 0.9.0.0-M21)
    To: gov.nasa.jpl.imce.oml.viewpoint [0.9.0.0-M21]

To reproduce, install M20, and update to M21.