Lambda is introduced as the one who tortured Bern in a labyrinth of despair until Bern awakened as a witch. It all being this very personal experience between Rika and Satoko just gives it much more meaning than an ancient witch randomly torturing a girl that had nothing to do with her - and then the girl
bonds with the witch afterwards.
It also explains why Bern hardly seems to blame Lambda for that specifically and seems to focus her grudge on Featherine. Truthfully, I think it's impossible R07 planned this from the start, considering how any direct Satoko hints in Umineko only appear by episodes 7/8, and Memoirs of Lambda, which was written around episode 3/4 to show she definitely wasn't Takano, something R07 mentioned in interviews at the time too, just makes any connection to Satoko odder.
However, many of the early lines about Bern's backstory just work much better with the set up seen in Gou, like Bern's reference to how someone troublesome taking a liking to Battler was similar to what happened to her when she was human, or how she could understand Lambda in their game, but can't understand Beatrice, or the times where Lambda references some kind of betrayal from Bernkastel in the past.
Maybe the basic idea always was something vaguely Gou-like but Lambda was initially supposed to be an OC rather than Satoko.