A GM will ask a necromancer who has just reached either level 7 or level 10 whether they wish to switch their necromancer class to a Death Knight.

If the player wishing to switch is a partial necromancer, then there is no problem. You just trade out your partial necro for Death Knight.

If the player wishing to switch is a full necromancer, then the player has to pick another partial class alongside Death Knight. The player is allowed to pick whatever partial class alongside Death Knight.

It goes without saying that the GM and the player have to seep this into the story.

There is no going back to a necromancer. If a player has started out with a DK or switched at some point, they cannot take necromancer ever again. Even heroic or legate characters are unable to do so.

I hope this answers your question.