Changing secondary

Once upon time i got a Ele/Nec now i changed to Ele/Mo if i change again in Ele/Nec i regain all my necro skill or they are lose forever?

(sorry for my english)

You keep all the skills on the character. If you started the game as E/N and goto E/M, all the skills you had as E/N will still be there for you to use. (Once you change back to E/N)