if we have already applied for visa in a particular state under 489 subclass, and then we receive another nomination approval from another state for subclass 190, is it possible to switch the state and subclass after applying.
I don't think so. You'll have to withdraw the 489 and apply afresh for the 190. Write to DIBP to check for your options/ possibilities and if none available, ask about possibility of a refund for your 489 upon withdrawal in case if it is not already allocated (slim chances of a refund though, but worth trying).
You cannot simply change the existing 489 application to a 190 application, you need to submit a new application for the 190. You can leave both applications in process but if the 190 is granted first, then immediately withdraw the 489 application to avoid it being granted and replacing the 190. By leaving both applications in process, you have a back-up option in case one of them is refused.