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SeaDAS has many tools for mapping. Image quality will suffer in regions where you are downsampling depending on the degree of downsampling and how different the pixel ground surface projection differs between the input and remapped file. Upsampling is essentially the manufacturing of data, so in this case image quality will likely be more blurred when viewed zoomed in at the new higher resolution pixel level.
l2gen will only output 20m resolution as Sean explained earlier in this thread. There are not enough bands at 10m for l2gen to do any useful ocean calculations. Sorry, but we do not have any plans to output 10m resolution for Sentinel-2 MSI.