Well, this thought has come to my mind so I said why not suggest it? I think members should be able to subscribe for an upgrade perk for a specific amount of time depending on how much credits and what is the perk. It will not just make people able to feel what it feels like to be upgraded and then give them thoughts about upgrading, but it will also raise the forum's activity because members will more likely start posting more and more to earn credits to subscribe for a perk.
Here's an example:- __________________ 1 week subscriptions:- Prime - 1,500 credits. Supreme - 5,000 credits.