TLDR (yet), but ...
A share is 'worth' 25 BTC/bitcoin_difficulty, at the time the share was found. It does not change when the bitcoin difficulty changes.
Need some rephrasing to make it more clear ... probably
A share is 'worth' 25 BTC/bitcoin_difficulty, at the time the share was found and it's value does not change later when the bitcoin difficulty changes, but only for the shares found afterwards.
Assuming his sentence is the first quote, and your proposed change is the second, his is far clearer. If anything needs to be added at all, perhaps the second sentence could read, "The 'worth' of the previously found share does not change when the bitcoin difficulty changes."
I just changed it anyway, made that section more detailed.