Swapping main breaker in a 200A panel with a 100A main breaker
You probably could do a breaker swap, assuming that a 100A breaker in the same case style as the 200A breaker could be found. I can't point to a manufacturer or product family, but I can at least say that Square D has decent drawings and technical specs available on their web site. Several of the drawings do indicate the case style of the main breaker, and some panels must be field-fitted with a main, so many of the drawings do include the information you'd be needing to match.
There are (at least) two other ways you could crack this nut. Supposing that your meter is located elsewhere, ie that you're not looking at meter-main combination type panels:

Install the new 200A panel and a 100A panel also. Set it up so that the 100A is the main panel fed by the existing service and call the 200A panel a subpanel. Later, when you upgrade the underground service, remove the 100A panel and have the upgraded service connect directly to the 200A panel. There's nothing wrong with a subpanel being equipped with a main breaker that's rated higher than some upstream breaker. If you find a 100A panel with output lugs you could wire the subpanel feeder directly from those to the 200A panel; no branch circuit breaker needed.
Install the new 200A panel with a 100A branch breaker rated for backfeed. Connect the service conductors to that branch breaker and mark it as the main breaker. Note: you'll probably be required to get a bolt-down bracket to retain the 100A breaker in the panel.