Maybe for bannings, but my extra slot issue was taken care of within a few days. But based on experience you are most likely going to have to purchase the merc 1 and 3 packs to get 8.
Maybe the original poster should suggest that anet give them those packs in exchange for the 8-pack since their goal was 8 slots and not their fault buying the 8-pack suddenly isn't working if you already had a 1-pack slot in place. It has in the past with people I know that upgraded.
Anyway worth the shot when you contact them. Probably won't work but worth the try cause that is the fair thing for anet to do.