If you were previously subscribed to $20, you may need to unsubscribe and wait until the end of the billing period to re-subscribe with the new price.
Sapphire-tier (ChatGPT-3.5) subscription lowered to $15 per month
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Source: Steam Community (opens in a new tab)
If you were previously subscribed to $20, you may need to unsubscribe and wait until the end of the billing period to re-subscribe with the new price.