i am goin to be switching over my 12g into a 24g cube , i was wondering if i add new live sand instead o my old live sand to the 24g can i just add all my corals, rock and ish or will the new live sand cause an ammonia spike?

and New DRY (NOT alive) sand and your transfer should go by without a hitch.

what he ^^ said is good
it will just be like a huge water cange
only u are put 200% back in

so its best to get dry, dead sand , instead of buying new live sand?

It's not better; it's just that bagged "live sand" is not very live, and so it's not a great benefit compared to dry sand when you add live rock.

Before you do the swap drain as much water as possible out of the 12g into buckets. Try not to stir things up so this is fairly clear water. Use this water in the new setup so your not doing 100% water change.

You might have already been planning on this but thought I would mention it just in case......