Certainly, Digium could grant a license for future commercial use of their code. Changing the nature of a license on previously released code may be problematic if the GPL2 terms already have been triggered.
Prefacing everything with "IANAL":
We discovered that the Grandstream UCM6100 user interface appeared to be based on the Asterisk GUI as well. Grandstream does not have a commercial license from Digium to distribute a derived work of the Asterisk GUI outside of the GPLv2; hence, it is our position that they have to abide by the GPLv2 for the "program", i.e., the user interface for the UCM6100. We have contacted them regarding this issue; I've received verbal confirmation that they will make the necessary corrections.