I guess Star Vijay already has HD feed. What might happen is that they will shut down SD feed and continue with just HD feed wherein SD viewers will get downscaled version of the HD Channel or quite possible both feeds will continue and they took this step as most of competitors had made a transition earlier hence they wanted to offer similar viewing experience.
I just hope they replicate this strategy for their Hindi channels too, very high chance that their Hindi GEC chs will adopt these changes soon as Viacom18, Sony have done so long back. In other genres like Hindi Movies or Infotainment or Kids i do not see this happening as of now or maybe star/disney will surprise us and make a transition in these genres too
Maybe Vijay TV noticed most Tamilnadu people use modern television which is 16:9, whereas their sd feed is widescreen which is so bad to see in modern tv but good in square shaped tv or something... Now TN people can enjoy sd feed like hd feed...