Yes you can replay any mission that you have already completed.Open the map of the Throne world via the destinations screen there will be 2 icons for the campaign at the bottom saying classic and legendary.Select which one you want and then where it says The Arrival click on that(not on launch) and you will be able to pick which mission you want to do.

Okay, I just discovered out how to tell which missions you have or have not Completed on Legendary, on a per character basis. This is on Xbox, so it may be slightly different for PSN or Windows. 1) Load the Character up in the game. 2) Go the "Director", click on "Destination" and select "Savathun's Thrown World" location. 3) On the Savathun Map look for the "Replayable Missions" text (below Fynch's location) and then click on the "Crowned Skull" Legendary icon. 4) When the "Legendary" windows loads up, click on "The Arrival" mission button. 5) The "Select Mode" window will load up with the list of all the missions on the left hand side. 6) If you move the Selector Cursor over each mission, the right hand side will list the "Mission Modifiers" for it. 7) If you have not completed a mission on Legendary yet, you will see a "Red Skull" and the words "A Challenge Awaits" for it under the "Mission Modifier" list. Select it and then "Launch" to play it. 8) Repeat the above steps to locate and play through each mission on Legendary until you have completed all of them. If you have already beaten campaign, then to get your Legendary Rewards from Ikora, you need to see her in the Tower, not on Mars. Hope that helps someone..