Generally yes. Assuming there isn’t some unique circumstance to your buying method, distributor, or country that I don’t know about Flex subscriptions are really a commitment of money over a period of time - not specific SKUs and quantities. You can change/add/remove entitlement as needed so long as the net effect of the change/modify order is the same or more money to Cisco (i.e., not a down sell). This is also how customers can move from on-prem to cloud mid-subscription.
