You should really talk this with your Qlik Sales person since licensing conditions change rapidly.
IAS is a different type of Server. It is not included with the Enterprise edition. In this case you will need 2 different Server licenses (your SMV plus IAS) since IAS is limited to one QlikView document, only works in Anonymous mode and requires a self developed user interface.
just to add to what's been already said, I think having a Small Business Edition and purchasing and IAS should not interfere in any way, mainly because they can not interact directly. SMB or enterprise work with Cals, while IAS only works with anonymous access only (So the concept of Cals doesn't exist for this type of server). So in case you acquire a IAS, you'll have to set up an enviroment aside from your "regular" QlikView environment.
So answering your question, I think there's no need to upgrade your SMB in order to have an IAS,