Just to be clear, Green-Flame Blade does not add 2d8 fire at 5th level. It only adds an extra 1d8 fire to the primary target and 1d8 + (spell ability modifier) fire to the secondary. You may have thought that since other SCAG cantrips such as Lightning Lure which increases to 2d8 lightning and Sword Burst which increases to 2d6 that Green-Flame Blade would do the same but it does not.
It increases to 2d8 fire when you reach 11th level. Booming Blade is also handled the same way as Green-Flame Blade as it provides an extra 1d8 thunder at 5th level and then extra 2d8 thunder at 11th level. The extra is in addition to the weapon damage done when casting and hitting with those spells (Green-Flame Blade and Booming Blade). SCAG p.143 (Green-Flame Blade) & p.142 (Booming Blade)
Best Regards
-Talo