Comment 2 for bug 559854

I think the best way would be to instantly drop out all the wares at the building flag. The carriers will pick up them. This way the upgrade wouldn't slow down.
If the flag at the building is full, then the wares that exceed could be lost.
Would it be difficult to implement?

Another idea.
If the upgrading building will use some of the same wares of the old one, these wares could be stored inside the upgrading building. Only unused wares should be ejected.