I think they have a franchise for x years then swap. One near us has been a Texaco, BP and is now a Shell. Each time they seem to get new pumps, new signs, new jetwash things and a better shop.
Depends which brand gives the the best costs. Like car insurance fuel resellers will offer a low price incentive and then screw over so people will change brands.
The once BP stations around my way turned into Harvester and have now all gone over to Jet in the past month or two (with the one just off the A1 at the A689 junction having the proverbial hacksaw taken to it).