You can re-use the same seed. I did that for the Frost Caves update. Be advised that minor things will be different, boss locations might move (a bit), but other than that, the seed should be very similar.

Edit: Ok, I think we need to discern between WORLD and SEED. A seed is... just a seed, a string of letters and numbers that can be used to generate a world.

A world is the actual world you play in that was generated according to a specific seed. In order to get the most out of the frost cave update, I generated a NEW WORLD with the SAME SEED as my old world.

I hope this makes it clear.