That's not really the intention behind the initial concept. If you are able to update a plane to make it more efficient while keeping the same behaviour and the same interfaces for the pilot, then it is great and there are no safety concerns with that.

The only issue on the Max is that as part of the refresh, they were not able to keep the same behaviour. And then, instead of fixing it with additional engineering or requiring new training, they hid that under the carpet and made sure the FAA accept itq, leading to crashes and deaths. Plus the denial after the first crash was really disgusting.