Toyota did not make changes to the engine. It's been documented in several writeups and videos that they tore down the engine, noted concerns, did further testing, and found that the engine met all their requirements for reliability. You have to remember that the B58 was well along in development independent of the Z4/Supra project, and B58 variants have been out in production vehicles for several years now. Whether Toyota could have changed the engine is another debate, but in this case they did not.I know Toyota improved internals on this engine. Had parts tested to meet its quality control. Reliability was a concern and BMW doesn’t have near the reliability of Toyota.
In the context of this discussion it means that if you have concerns about BMW reliability then those concerns will be the same in the case of the Supra. However, the fact that it passed Toyota's rigorous examination and testing is a good sign for both brands.