Upgrades applied to your Fortnite Founder's Pack purchase add the difference between your previously owned Founder's Pack, and the tier to which you have just upgraded.
A Standard Founder's Pack includes 4 Exclusive In-Game Banner Icons.
A Deluxe Founder's Pack includes 10 Exclusive In-Game Banner Icons.
A Standard-to-Deluxe Upgrade will grant 6 Exclusive In-Game Banner Icons, which brings you in line with what is included with the Deluxe Founder's Pack.
If you are simply trying to locate your Founder's Pack content, please refer to this related Support Article.