I am living in an "old" house with 100 amps coming in. I am in the process of doing reconstruction and would like to know if I run all new wires and outlets and get rid of dusty broken wires and connection boxes left open to rot---Can I use the same fuses and wiring for 100 as for 200 or do I rewire everything in a new box etc and then switch? I cannot see us going without power for a prolonged period.

The old 100 box may not have enough spaces in it to hook up all the circuits you want. And if you buy a new larger box sometimes different brands of circuit breakers will only fit a particular brand of box. So it would probably be easier to get a whole new set

If you are re-wiring that much of the house you'd be crazy to not upgrade to 200A.

Why do you think you will be out of power for very long??

If Materials and Tools are on hand and prepared Correctly, You should have a Minimum Down Time. Have not run into one that can't be done in a day. If many solid metal conduits
Enter the Top and Bottom of Panel Is MUCH more of a Job than say Romex.
All solid from the Top OR Bottom is no so bad.

If a DYI is doing a service upgrade, then it is very likely they will be without power for over a day.

Just my .02 ;)

Well, that's what generator rentals and pizza are for.

Send the wife and kids to a motel with a pool while you tough it out alone. And if you have all the wires there at the fuse box in a workman like manner and ready to go it's not so bad, a lot easier than trying to squeeze new wires into an existing box.