replacing a mobile home, what do I need to know beforehand?

we are trading our double wide in for a newer and bigger double wide.
We get conflicting answers regarding building permits and electric hookups.

Since its on our land in which our current mobile sits, water, electric and septic are here and are in use.

Do we need to contact the electric company to disconnect the old mobile and hook up the new one?

Secondly, do we need a building permit when replacing a old mobile with a new one?

And third, how long does it take to set up a mobile? One person said it can be done in a weekend. Another said it takes almost two weeks, between setting footers, and getting the mobile completely together.