This was not dipping a toe into the waters of a rebuild. This was diving in headfirst. Just as the Rangers said they would, they have embraced a full overhaul of their roster, and went through their most dramatic leap just moments before Monday’s 3 p.m. trade deadline. That’s when they traded captain Ryan McDonagh and forward J.T. Miller to the Lightning in exchange for a 2018 first-round pick, a conditional 2019 first-round pick, young Lightning forward Vladislav Namestnikov and two prospects in 19-year-old center Brett Howden and 20-year-old defenseman Libor Hajek. Surely it wasn’t easy for general manager Jeff Gorton to trade away McDonagh, who is 28 years old and has one more year left on his contract carrying a $4.7 million salary-cap hit. Same for the 24-year-old Miller, who will be a restricted free agent. But now that the Rangers have traded their captain to the Lightning for the second time in the past four years — to go along with Ryan Callahan for Martin St. Louis in 2014 — the process that Gorton and team president Glen Sather promised in a letter to the fans back on Feb. 8 is coming to fruition. “It feels different as far as we’ve had a lot of players leave in a short period of time,” Gorton said on a conference call Monday evening. “But when we sat down with our plan and what we wanted to do and started talking to teams with where we were headed and some of the things we wanted to accomplish, you started to get a feel that this is going to happen.” Gorton spent the earlier part of the week trading away his prime rental players in Rick Nash, Michael Grabner and Nick Holden. In doing so, he has now amassed 10 picks in this upcoming draft, including three first-rounders, with the two coming from Boston and Tampa Bay set to be later in the round but still extremely valuable assets. The condition on the 2019 second-rounder is if the Lightning win the Stanley Cup this year or next, it becomes a first-rounder.