If S3D see the stl file has changed and you choose to update - the existing support structures stay on the build plate and the updated geometry is placed beside it. The support should be automatically removed.
Dense infill is a great idea but many patterns make no sense. The dense infill will simply drape itself into the void of the primary infill. In my case I would like to use the fast honeycomb as primary pattern but have grid as dense infill because it bridges better.