As you may be aware, we have covered the upcoming 31 October 2018 TLS 1.0/1.1 support being removed from O365. You can find that guidance here. As promised, Microsoft has finally published the last pieces of the series.
As usual, I strongly recommend that you start looking at this now – some of this might take a bit of planning and coordination on your part to accomplish in a clean fashion.
Now is the time to get your sales types contacting existing customers and offering to help. This will not be a clean thing; rest assured that there will be “issues” and IMHO there is significant potential for unintended consequences.