If I am initially charge with a DWI and the blood alcohol comes back below the limit, can the charge be changed to a DUI?

You generally are not charged with DWI or DUI until the officer peforms the preliminary breath test (which gives them the probable cause to arrest), and then the datamaster test (the official test) or blood draw to determine you actual blood alcohol level, which is usually done after two PBT's are performed. The latter is what the your charges are based on. So if you initially blew a .08, then the actual datamaster showed .07 or lower, I would challenge the initial charges of DWI. However, keep in mind that you can still be charged with DWI depending on how much time elapsed between the initial PBT and the Datamaster test or blood draw.