What I heard from someone on another board was that Yamaha has no immediate plans to release the PSR-E453, because the PSR-E443 is still selling well-- and if or when they do release it, it might even be called something else. Generally speaking, Yamaha seems to be moving toward a longer interval between successive models of a particular line-- e.g., a line that used to get a new model every 2 years might now get a new model every 3 years instead.
Personally, I think it's odd that there was no PSR-E453 announced this year at the Winter NAMM, because someone had posted some information in Wikipedia last year purporting to be about new styles on the PSR-E453. There was no reason to believe that the information was accurate, but if it was based on some sort of insider info then it meant that Yamaha was working on getting the PSR-E453 ready for market. My own theory is that Yamaha decided not to release it at the Winter NAMM because they had other keyboards they wanted to focus on-- namely, the three MONTAGE models, as well as the DGX-660-- so they thought it would be better to wait several more months until Summer NAMM to announce the PSR-E453. I guess we'll just have to wait and see what happens later this year.