You will have to get the experience points, it doesn't matter how high your level is.
Here is a reference, this will definitely help:
http://www.bungie.net/Stats/Halo3/RankHistory.aspx?player=StS%20Precision
This is an example of a great player, who leveled up very quickly. But he hasn't played many games so his Ranking as a lieutenant hasn't changed.
It shows he has gone up to a general, but that is only for the level. It is still displayed that he is a lieutenant to everyone else.
I hope this helped.