There should be no issue with migrating libraries that have checked out objects. We recently upgraded from PB12.5 to PB17 under that exact scenario, and I would imagine that things would go as smoothly with PB11.5 as with PB12.5.
After the migration, developers with checked out code, checked out their objects in the migrated PB17 code, applied their changes, and they were good to go. We use the PB Native SCC, and version label our SCC folder structure, so that we could run the PB12.5 version side with the migrated PB17 version, making it easy to keep track of which objects in PB12.5 were new or checked out. With PB's Edit Source capability, it is very easy to copy objects individually from PB12.5 to PB17.
