>>13347392Possibly, but swapping engine infrastructure completely would be a nightmare, even SpaceX haven't done something like that. In a lot of vital ways the engines are the heart of the launch system, a lot of other design considerations revolve around which engines you'll be using.
You wouldn't want to be say, swap Merlins for RD-180's or visa-versa, same for swapping Raptors in for BE-4s, even though Raptor is a far superior engine in pretty much all regards.
For example Vulcan's thrust puck is going to be configured to mount specifically BE4's, and only needs to take up their pathetic, filthy, soot filled thrust. With two Raptors to every one BE4 they'd need twice as many plumbing apertures and an uprated thrust puck since a Raptor is more than .5 BE4's in terms of output. This might not be an issue if ULA worked like SpaceX does, but at this point it would probably take them years to change course on thrust puck design, and that goes for every other system in the rocket involved with engines.
