Option for viewers to upgrade sub tier for a specific time period
Allow viewers to upgrade to a tier 2 or tier 3 sub for a specified time. For example, upgrade to tier 3 for a single month to celebrate a special occasion, or just as extra thanks to a streamer, even if the viewer isn't able to sustain a more expensive subscription. Allowing a set time lowers the reluctance to try out higher tier subs for the fear of forgetting to lower the tier back again later.

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Jimorian commented
I did think of an external complication due to recurring billing policies of 3rd party payment providers and banks in that changing the billing amount would still probably need a manual re-authorization of the amount. This could possible be worked around by creating a 2nd payment for the difference that's only valid for the set time period.
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Stennos commented
And allowing someone to gift higher tier perms for a month as well. So I can gift a mod Tier 3 for the month, even though they have a running Tier 1. After the 30 days is up, they drop down to their Tier 1 perms
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Trabbitt commented
Unable to upgrade sub tier when subbed using mobile