The problem with those seats that convert to a booster is that most of them only harness to 40lbs. The only one I can think of off the top of my head that goes higher than that is the Cosco Apex65. It harnesses to 65lbs. and then is a belt positioning booster until 100lbs. The only requirement is that your vehicle have adjustable headrests or very high headrests that can go behind the carseat as it is not reinforced.
Just out of curiosity, what seats are they in now? I saw you mention Britax and wondered if that's the seats they are in now. They harness to 65lbs.
Btw, good for you for wanting to keep them harnessed longer. The longer you can keep them harnessed the safer they will be.