In the majority of places with O365 I have always noticed that the Site Owners group and site members have their permission locked to Full control and Edit because they are in sync with the permissions of the Inbox annexed. It is not possible to downgrade Edit to Contribute.
However 6 months ago I was in a situation with an enterprise license where I was able to do it. Has anybody experienced the same? It did not seem to me that there were repercussions on the integrity of the inbox if I downgraded Edit to Contribute.