Board Thread:Questions and Answers/@comment-29255713-20160723231516/@comment-27701762-20160723232646

If you've already defeated Asgore, then you can choose to spare or kill him again. If you spare him, he gives different dialogue and kills himself to let you free, but Flowey destroys the soul as normal.

Flowey does not transform back into Photoshop Flowey, as he knows you will win, so he just skips over it all, meaning you don't get to do the boss fight or the whole spare sequence to get advice again.