@Original Raw and Uncut, but they wouldn’t really be doing away with SDTVs. To fill in the former space, Nick would start using the HD feed downscaled to 4:3, which would result in all shows to be in the 16:9 ratio. On the HD feed (eventually being used downscaled for the SD feed), for 4:3 shows (SpongeBob for example), there would be blue bars on the left and right, and would look like here
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And this is how they would look like on the SD feed after the original one was retired and the HD feed downscaled would be used
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Something like what Cartoon Network did back in May 2013.