Among those changes are the grille and the wheels. The lower outboard grilles feature a chrome crossbar and a vertical chrome accent. On the M850i coupe and the convertible prototypes we've seen, the crossbar is absent. The wheels are of a new multispoke design that isn't available on the M850i coupe as a standard or optional feature. They haven't been seen on convertible prototypes, either.
These changes may suggest this is a different trim level of 8 Series, maybe even with a different engine than the twin-turbocharged V8 of the M850i. But we can't say for sure, since contradicting that idea is a taped-over badge on this convertible's fender that's in the same place and roughly the same size as the M850i's M badge. Also, the rear fascia of the convertible is identical to the M850i convertible, down to the parallelogram-shaped exhaust tips. As such, it's also possible these cosmetic changes are just ways to differentiate an M850i convertible from the coupe.
Since BMW just revealed the 2019 8 Series coupe this year, we're expecting the convertible to follow next year. This is also likely considering that BMW has been testing the car simultaneously with the coupe, and this latest version is almost completely uncovered in the coupe's debut light blue color. It will probably offer the same engines as the coupe, meaning that it will have at least the V8 from the M850i, and possibly a lower-power engine option not long after launch.