I imagine you can use modifiers and just change the number of wall loops from 2 walls to 1 wall and the applicable layer. There was a similar question about changing infill percentage at a certain layer height and I provided the following workflow for that
I was in a similar situation yesterday - I struggled a bit that the option from PrusaSlicer ‘Height Range Modifier’ is not available here. Modification models can be a strong option but a bit to much of an setup (and I still struggle to position them correctly).
But in my opinion an ‘easy’ alternative is to ‘CUT’ the model at the desired height (e.g. 5mm) without any rotation (Cut Tool | Bambu Lab Wiki) with the option ‘cut in parts’. In that way it keeps the model intact. Then you can adapt several of the setting (including infill, nr of top and bottom layers, perimeters) independent from each other. You have only to switch into the ‘object’ tab.
Hope that helps - it is relatively easy once you get a hang on it.