for _, instance in pairs(game.Workspace:GetDescendants()) do
if instance:IsA("BasePart") and instance.Transparency == 1 then
instance.Transparency = 0.5
end
end
If you pop that into a Character as a LocalScript, every part in the workspace with a transparency 1 will get a new transparency of 0.5.
However, some parts (like the HumanoidRootPart of characters) should probably stay fully transparent. Therefore, you'll want to add some checks to make sure you're targeting the parts you actually want to change. You can accomplish this any number of ways (eg. where the part is in the workspace, tagging the part with a tag value, or tagging the part using CollectionService).