Christchurch were offered ‘upward movement’ based on their PPG over the last two unfinished seasons. They declined (apparently the only team in the country to do so), so it went to the next best PPG performing team from the Wessex Prem, which was Lymington.
It’s the same process which has seen United Services Portsmouth & others promoted from Wessex One to the Wessex Prem (with others) and Infinity & Bush Hill promoted to Wessex One.
You might have also missed Moneyfields moving back down into the Wessex Prem by their own request. That division has loads of Pompey derby’s next season.