This is a good question. Synching is allowed for Element templates, but not for feature definitions. I agree, this would be a good improvement. To add to your request, I'd like a way to run a report on my files to tell me which dgn's have feature definitions that are out-of-sync from my dgnlibs.
In the meantime, perhaps this workaround will help?....
open your dgn with the feature definition to update.
copy the feature definition from your dgnlib to your active dgn. Since these have the same name, a "1" should be added to the end of the copied name.
Use project explorer to toggle off all elements based on feature definition only leaving on the one you want to update.
Use microstation to select these elements.
Use openroads set feature definition to change to the copied feature definition (with the "1")
delete the original feature definition name.
rename the feature definition with the "1" to remove the "1".
In the meantime, perhaps this workaround will help?....
open your dgn with the feature definition to update.
copy the feature definition from your dgnlib to your active dgn. Since these have the same name, a "1" should be added to the end of the copied name.
Use project explorer to toggle off all elements based on feature definition only leaving on the one you want to update.
Use microstation to select these elements.
Use openroads set feature definition to change to the copied feature definition (with the "1")
delete the original feature definition name.
rename the feature definition with the "1" to remove the "1".