In most cases, it is not possible to upgrade the software on a switch to a completely different type of software or Operating System.
In the case of the Legacy AT-9000 switch, it is possible to upgrade from the AT-S100 software to AlliedWare Plus. In fact, it is highly recommended to do so. Upgrading the software to AlliedWare Plus gives more support for new modules.
The process for upgrading can be difficult. Here is a link to the upgrading process. http://alliedtelesis.com/media/datasheets/srn/AT9000_AWPlus_v211_SRN.pdf Please Note:
An active Net.Cover Support Contract is required to obtain any software updates.