Go to nirsoft.net and download nk2edit. Add this to your toolkit, you will need it everytime from now on.
In the past you could rename the old Autocomplete with the new name and it would work. Now it seems to recreate the new one leaving you with a blank.
So open nk2edit, it will show you the current autocomplete. Open another nk2edit and in that one, open the old autocomplete (change the nk2 file option to 2010 autocomplete).
Select all the entries in the old one, copy and paste into the new one. Save, re-open outlook.
Autocomplete should now have all the old entries
Another way to do it is to open the old autocomplete and export to message store.
I’ve had various results with the different versions of outlook and exchange and O365 accounts seem to do something different.