Tonight’s guest host is Jim Parsons, and the musical guest is Beck. The most notable new participant in the long run, though, will be Colin Jost. He’s been with Saturday Night Live for years, serving most recently as the head writer – but starting tonight, he moves to the on-camera side of the TV lens, as Cicely Strong’s new co-host on “Weekend Update.” He may not always be credited as such, but Jost really is the final domino to fall in NBC’s current late-night shuffle: Jimmy Fallon moves from Late Night to Tonight to replace Jay Leno, Seth Meyers moves from SNL to Late Night to replace Jimmy Fallon, and, starting tonight, Colin Jost moves to “Weekend Update” to replace Seth Meyers. Other SNL head writers who have gone on to host “Update”? Well, just for starters, there’s Seth Meyers… and Tina Fey.