Britax has a free replacement policy for carseats that have been involved in a crash: their website has detailed info on that for you to find out if you qualify -- it may not need replacing, but if it does you might as get it done for free

Or, you could contact your insurance company & if they deny your request ask for such in writing (they usually change their minds then) because damage usually cannot be seen with the naked eye....
If I were you, I'd consider the Britax
Regent. It'll keep kiddo harnessed the longest (which is safest) & you can hand it down to baby when the
Roundabout does expire

Your bigger kiddo will likely still need a
booster afterwards, but boosters are improving so much there should be some great options by then!
Another idea is to use the EZOn/SafetyAngel Kid-Y with any dedicated booster of your choice. This would be good for travel as well, just make sure that the lap belt is snugly fastened before tightening the harness. Here's another discussion about this option >
http://www.car-seat.org/showthread.php?t=19056 (I would consider this a lower priority option compared to the Regent)
Both of these options require
top tether anchorage.
Another option is the
Recaro Young Sport. It is not inexpensive & has lower harness limits, but it does become one of the most dedicated boosters & is very comfortable. Recaro is a name known by many dads as the reputable racing seat designer
I personally have Husky (old name for Regent) + a Cosco Apex & have used the Kid-Y & RYSport in the past, plan to purchase a new Kid-Y to use with our folding
Compass B510 booster this winter in our plow truck. The Apex does not keep kids harnessed as long as these other options & is rather flimsy so I keep it in our lesser used vehicle & for use on airplanes. The RYSport & Regent are by far our favorites.