In my current favourite layout, Mississippi covers the lands from 32°25' N up to 35° N stretching from the river itself to the Georgian border, so your bit of the OTL state (Tupelo?) is still part of this Mississippi.
The lands south of 32°25' to the sea are a successor to the West Florida province, and tend to be called either West Florida, Jackson or Washington depending on how the states come about, what order the trans-Appalachian states are created in and whether Washington becomes a renowned separatist leader or not.