Complex sentence: Under the original plan, the Titan IV would only be paired with Centaur stages and fly exclusively from LC-40 at Cape Canaveral, but the post-Challenger program would also involve flying IUS (Integrated Upper Stage) or even no upper stages.
Split into simpler sentences: Under the original plan, the Titan IV would only be paired with Centaur stages and fly exclusively from LC-40 at Cape Canaveral. The post-Challenger program also involved flying IUS (Integrated Upper Stage) or even no upper stages.