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.