I'd actually like to know this too, because I don't really like my old thumbnail, but I left it rather than making a new one because I'd done my nice shiny cover and thought thumbnails were being retired.... aaaand now I've been featured three times on the front page in staff, community and a genre pick with my crappy old thumbnail.
I'm very happy to have been featured, but if I'd known we'd still be using thumbnails at this point, and being featured with our thumbnails, I'd definitely have made a new one. It looks absolute pants compared to those gorgeous, polished Tapas Original comics it's featured alongside right now. 
So, Tapas, is it a reasonable use of my time to make a new thumbnail or are they going soon? Square format work that needs to scale down to such a small size and represent the work in an eyecatching way really requires a bespoke illo for that one purpose to be effective (My cover is a group shot, so part of it won't work well composition-wise at that shape or size), so I'd want to spend time and effort on it if I was making a new one, but I don't want to spend all that time only for it to turn out they're really actually going away like... in two weeks or something. Would be very keen to know a rough timeline!