We did book under the buy 4 get 3 promotion... but how does that make a difference regarding ticket upgrades?
Yes. USE your current tickets at the park FIRST, then upgrade.
Why could we be better to wait until AFTER we use one day?
The "use a day before you upgrade" is important if
there is a difference between what you originally paid
for your current ticket (if you paid LESS) and what
that same ticket costs at the gate the day you
upgrade.
This would be in the case of a ticket that you bought
from Disney before the price increase, with a special promotion,
or a ticket that
you bought from a discounter.
BEFORE you use a new ticket, and try to upgrade, that ticket is considered to be a "trade-in."
So Disney will check to see what you paid for the ticket and charge you the difference from THAT price to the gate price of the upgraded ticket.
AFTER you use any part of a new ticket, and then do the upgrade, that ticket is considered to be a normal "upgrade."
So Disney will only charge you the difference price as if you had just purchased your current ticket at the gate and only charge the difference between that value and the upgraded ticket.
And it's so easy to use the ticket first, anyway.
Then, at some later point in your trip (say, "day
two,") when you take a couple of minutes' break, do
the upgrade.
FWIW, don't WORRY about WHY you should use a ticket first.
Just know that it is the only way you can save the discount that you may have gotten.