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.