After going through the entire thread, may I summarize it as the following:

For Air France operated TATL:
- for flights originating in the US, one can upgrade from a Delta purchased (006) ticket with Delta flight number in W class to O class. This can be done by phoning Delta. The actual procedures that need to happen on Delta's side is not important, but it basically involves Delta changing the ticket to AF flight number and processing the upgrade. This is clear.

- for flights originating either in the US or in Europe, once can always upgrade from an AF booking class of S or W into O, regardless of whether this initial S or W class ticket was purchased from AF or from Delta (is this correct that the ticket does NOT have to be purchased from Delta?) The upgrade needs to be done by phoning Delta since you're using Delta SWU/miles. AF A class is not eligible for upgrades.

- for flights originating outside the US (aka, originating in Europe), Delta does not have the ability to change a Delta (006) ticket/flight number W class ticket to an AF flight number. And hence, a Delta W class originating outside the US is NOT possible to upgrade. Is this understanding correct? If so, this would mean that for those originating outside the US, the only way to upgrade would be to buy the more expensive AF S or W fare.

Can people comment on this? I am hoping that this will provide a useful summary for reference.