Hard --> Easy:
Soaring Guardian God - At first I was trying to focus on dodging the bullets using SakuyaA, so the spell card took forever to finish off, and I'd always end up having to bomb sooner or later. Then I switched shot types, stayed under Chen, and now it's over quickly and a total cinch.
Princess Tenko -Illusion- - Ditto, only it's mostly because the waves speed up.
Unilateral Contact - Exactly the same as Princess Tenko.
Kokkuri-san's Contact - Something clicked and now I can slip in between bullets with ease.
Yukari's Arcanum "Danmaku Bounded Field" - Realized it's static.
Past "Old History" - Realized the small bullets are static.
Reincarnation "Returning-Bridge Ichijo" - Not sure how or why on this one.
Hourai Doll - Something clicked, and now I can keep track of the round bullets and the yellow and red bullets at the same time. I used to run into one kind trying to dodge the other.
Kaguya's third Last Spell - I realized this was easier to sit on the bottom and dodge horizontally with minimal vertical movement.
Easy --> Hard:
Lost Soul's Village - Trying to stay under Yuyuko makes this one HARDER.
Perfect Cherry Blossom of Sumizome - The very first time I ever faced this card (on Easy), I was out of bombs, and by some crazy fluke, I managed to capture it. I think I had an adrenaline-induced bullet time effect. To date, that is the only time I've ever captured it, mostly 'cause now I always have bombs left, and I panic and bomb easily.
Easy --> Hard --> Easy:
Mesh of Light and Darkness - Due to the way the bullets move faster than the laser beams spawn. First time I faced this, I had a bad habit of retreating to the very bottom at the slightest sign of danger during a boss battle, which caused the bullets to pass me before the lasers fired. Then I broke that habit, which was good for most other spell cards, but I'd be just far up enough that the lasers an bullets would hit at the same time. Then I figured out WHY it suddenly became harder, retreated back down just for that card, and it's easy again.