Once a new series is added to the game, it isnít removed but your alts may not be able to play the missions. The missions while the FE are released are at lower lvls but like the last one it is now in a higher level range unless you did the mission and unlocked it.
A new lt commander cannot play the mission now since it is now a captainís level mission. Not in the game so not sure the exact level it might even be a rear admiral mission. When they have rerun missions the level restriction will be lowered and any toon that did not do the mission can do it and have it unlocked for them.
Edit: on replay missions the level restriction is not lowered. You can only unlock it for lower levels if you play it during the initial launch.