> Will the change in the upvote in HF20 be retroactive for all posts that were already posted and voted, but less than 7 days old?

I assume not, but not 100% sure.

> Does HF20 mean that now the linear time factor increase of 0-30 minutes will decrease to 0-15 minutes?

From what I understand, the reverse auction is 30 minutes and if you vote at 14 mins 50 seconds, you will only return a tiny portion of your vote to the reward pool.

> What does it mean to return to the reward pool?

When a vote is cast, a certain amount of `rshares` are claimed from the reward pool, this represents how much that vote will be worth (which does adjust slightly with how the pool is being currently used).  With this change, anything in the first 15 minutes will not be deducted from the reward pool,

> For the vote to return to the reward pool of its post, or for it to return for the global reward pool, or for it to return to the posts' rewards part of the global reward pool?

Global reward pool is how I understood it.

I will check to see if I am wrong in these assumptions, but that's how I understand it right now.