According to a recent GAO report (p. 116), ULA's Vulcan rocket's BE-4 engines are "experiencing technical challenges" and there is some concern whether they will be ready / certified in time for the planned first launch.
Hypothetically, if ULA were to seek another engine to replace Blue Origin's BE-4 with minimal alterations to its existing hardware, the only high-thrust methalox engine I'm aware of that's more developed than the BE-4 is SpaceX's Raptor engine. Now, even if it were technically easy to swap in Raptors for BE-4s, there's no way either SpaceX or ULA would agree to such a deal.
But I am curious: if they somehow did agree to this, how easy would the swap be from a technical perspective? Would it be feasible to adapt to a Raptor plumbing designed to supply a BE-4? Would an additional Raptor be needed (the sea-level thrust is 2.2 MN for a Raptor vs 2.4 MN for a BE-4), and if so would there be any feasible way to add it (as I understand a fifth F1 engine was added to the Saturn V)? What other issues might arise (e.g., fuel mixture, fuel temperature, ...)?