As much as this may be beneficial to those who cannot afford or do not want to spend their money on forum upgrades, it beats the purpose of having upgrades.
Selling upgrades is a way for NulledBB to earn some cash for expenses such as domain renewals, hosting, etc, and probably personal expenses of the people running the site. As a bonus for helping NBB, upgraded members get cool perks depending on the amount they have given. Aside from that, credits cannot be purchased and are just earned by forum activities or betting. It also cannot be exchanged or converted to actual/digital currency. Therefore, credits do not have any real life value. Not to mention this may cause a possible rise of spam and LQ posts for the sake of credits. NBB will be at the losing end if they allow the exchanging of credits for upgrades.
Though, subscriptions may be good if it includes actual money. Say, $1 for a month of Prime subscription. This will be a win-win for the members as well as NBB peeps. $1 looks cheaper (not in the long run) so it will be more appealing to non-upgraded members. Aside from that, if a member chooses to keep subscribing per month for the whole year that will give NulledBB $12 for a whole year compared to a one-time, lifetime $6 Prime upgrade.
