At this time there is only a workaround. The workaround is to split out the amount of licenses that you want to upgrade. Then modify the count for licenses for the original number of license so the count is minus the licenses from the newly created license. Then you upgrade those licenses.
Here is a kb article that talks about this as well:
http://www.symantec.com/docs/TECH183228
There are also plans to add this functionality to a future release.