I have a Murray/Siemens LC3040B1200P circuit service panel I would like to use to replace my existing panel. The present service is 100amp and the new panel is 200amp. Is it possible to just change the main to a 100amp main and use the new panel? What is the new main number? Thanks...

You have a problem trying to use the new 200 Amp panel.

Best you go out and get a 100 amp rated panel with a 100 Amp Main Circuit Breaker.

There is no method of removing the 200 amp CB in your new panel and replacing with a 100 Amp CB.

Removing the 200 Amp main, or otherwise modifying the panel in any fashion, will violate the UL Listing of the panel.