I have Sketchup Pro 2018 on Mac. I need to work on a .skp model that was drawn in SketchUp for Web 2020. I had hoped to be able to use SketchUp for Web to open and Save As a v2018 file but I see there is no option to save as older version with SketchUp for Web. I also don’t see how to use SketchUp for Web to upload the model to 3D Warehouse in order to download it as a newer/older version. Eneroth open newer version plugin is PC only 
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‘Save as’ is to save a file in the cloud under another name or in another folder.
Thank you! Sketchup has devised an interesting way of interpreting the term “Save As” …
There’s no reason to set a version with Save as… since it saves to Trimble Connect which uses the most current version .skp file.