That is an absolute NO-NO! If you replace a 100 amp main breaker with a 150 amp you will have compromised the very reason that the 100 amp breaker was there in the first place! The primary reason for having a breaker in the first place is to protect the wires feeding the panel from overload which causes tremendous heat that can quickly start fires. I would counsel anyone that is not a trained electrician to seek help from a licensed electrical contractor or a friend that works in the business as an electrician to walk you through it so that you are safe, the house is safe, and anyone that lives in the house should you move will be safe.
Yes. Panel size is amounted spots in the panel. 100=less spots, 150 or 200= more spots. This keeps the Harry homeowner at bay from putting say 20/ 20amp breakers in the box and then cooking the service. just keep the 100 amp in there till your ready for new service into your home. Make sure you have enough room for the new cable to come in and loop to the box. Make sure the service is off before you pull the main legs through both boxes. Be safe on this one.....you ever hear of a plumber drowning?? Electricity can kill ya.