Notice Chris said he didn't touch the outside service. That's because it is remaining a 100 amp service. The 200 amp mains rated panel can be lowered to a 100 amp mains rated panel by installing a backfed breaker on the branch circuit breaker busses in the back fed position. This allows him to continue to use the 100 amp service entrance and be ready to up grade to 200 amps by having a panel already in place.
I see this has already been explained by theatretech.
Sometime I'll have to go to Rexell and have them show me some of this stuff before it is assembled and mounted and buttoned up.