Upgrading RTW - what date of the fare applies?
 

Let's say I buy an RTW ticket ex CAI - Economy Global Explorer MGLOB34 - if I understood the rules correctly, I can later upgrade it to a DGLOB34 without penalty.

My question now is:

If I buy the ticket April 2nd, there is a fare change April 15th and I do the upgrade June 1st, before the 1st leg of the RTW even starts - do I then pay the difference to the fare level on June 1st or to the fare of the date when I originally bought the ticket?

And I assume that I will always pay the difference to the D fare in the country where I originally bought the ticket, right, even if I buy the ticket in CAI, have it DHL'd to me, hold it in my hands and then upgrade it in, let's say LON?

Thanks for your advice!

You'll pay any price differential at the point that the fare is ticketed, rather than reserved.