How do you change Asset Groups on Assets

Our asset groups are intended to show Make and Model, however due to time pressures and lack of information at the time of creating Assets we created dummy asset groups instead.  For example, all our Fire Pumps are assigned to a dummy PUMP.FIRE asset group.  It turns out that not all Fire Pumps are the same make and model, so now we need to create new individual Asset Groups and replace the original asset groups on the existing assets.  Is this possible?  I am reluctant to Deactivate the assets as there are large numbers like this, in addition to existing open work orders,