Prorate subscription price based on subscription time not used when purchasing a subscription upgrade
If I sub to a user with a 6 month 4.99 sub and then during my third month decide that I want to upgrade my sub to 9.99 I think Twitch should charge me the difference of time remaining and then once my sub renews at the end of the initial 6 months, it would charge me for my plan at the new price.
In the case above, I have 3 months left unused in my sub. So for today, to upgrade my sub, I pay $15 to be brought up to the 9.99 tier for the next three months. Twitch and the streamer can split the difference depending on their arrangement. Then when that sub ends, I'd be charged the full price of a 6-month 9.99 sub when I renew.
As it is now, if I buy a bulk sub, I'm locked in at that level unless I want to lose my investment in that sub to upgrade. So, for anyone buying a 6 month sub, you are actually disincentivized to upgrade your sub.

We are working on some more robust multi-month tech and robust upgrade/downgrade tech. So when we get both done we’ll definitely incorporate this functionality.
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bri__happy commented
Any updates on the upgrade/downgrade tech?
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WiteKngt commented
I just noticed that I can't upgrade a subscription without paying for a whole new one, and saw that someone had asked about prorating subs, but that was nearly three years ago and it was mentioned by someone from Twitch that there was a plan to change that and also to add new tiers, but there have been zero updates.
Can someone from Twitch tell us whether or not this is going to happen, and if so, why it hasn't yet? Gym memberships can be prorated. Some things at the DMV can be prorated. Why won't Twitch do it? It seems rather awful to force people to pay full price to upgrade.
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WiteKngt commented
DJFLuFFKiNS, it's been almost three years! How long does it really take to put something like this in place? I want to upgrade on a few of my favorite streamers' channels, and I can't without paying for a whole new subscription!
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KensonPlays commented
Any updates on this? I'd love to be able to do this for streamers I enjoy a ton!