Boeing were even promoting the possibility that a 787 will happily and safely fly with a GE engine on one wing and a RR on the other. Boeing promoted this as useful (??) if say a GE powered 787 breaks down at or near to the base airport of an airline whose 787's are RR powered. A RR engine (or vice versa) could be loaned / rented for a few days to keep the a/c in service. In theory the owner of the stricken a/c would save a number of revenue days that would be lost by having to fly an engine out from their maintenance base which could be the other side of the world.

And all this is possible (claimed Boeing) because of common hardware interfaces to both engine types and more importantly, software for the engine control systems that can be reconfigured quickly to make different engine types behave identically. To the pilot, the whole thing will be transparent and he / she would not have to use any different procedures when dissimilar engines are fitted.

I have my doubts as to whether this is a real 787 benefit or not though.