Martin Lewis has said that Octopus has brought Shell and you will be automatically switched over. However if someone refers you from Octopus before this happens then you can split £100 between you both. Here's the article about it https://www.google.com/amp/s/www.chroniclelive.co.uk/news/cost-of-living/martin-lewis-shell-octopus-trick-28079188.amp
I was with Shell and my partner referred me to Octopus and Octopus gave £50 to me and £50 to him. So if you want to bet the switch over here's my referral code Use this link to sign up to Octopus - https://share.octopus.energy/brave-owl-448
You don't need to do anything once you fill the form in. Shell and Octopus sort the switch between themselves.