Again, these are the ones that I've found so far. Feel free to correct me and add on other ones I don't have.
2 Stars
I only have Mint evolved out of the 2 Stars.
-Violetta/Purple Pansy, Amaryllis (slight), and Mint got sprite updates. (Not sure about Cycla Menn, but I think she had a unique sprite before.)
3 Stars
I only have Jasmine, Blue Lotus, and Nymphaea evolved out of the 3 Stars.
-Nymphaea and Blue Lotus both got sprite and animation updates and an additional "Cheer" animation each. Blue Lotus also has an additional "Standby" animation.
4 Stars
I don't have Erythines yet, and I don't have the following evolved: Hibiscus, Dandelion, Cyani Cornflower, Japanese Rowan, and Sweatpea.
-Sprite updates: Everyone except St. Paulia, Yulan, Cepha Lanthera Lilac, Japanese Rowan, Zephyranthes, and Erythrines. (Canola's, Di Yu's, and Violetta/Purple Tulip's might not be immediately noticeable.)
-Additional "Standby" animation(s): Casablanca, Snowflake, all 3 Tulip sisters (Violetta/Purple Tulip might be hard to notice.)
-Additional "Move" animation(s): So far, I only see one for Canola, but it's only that she hold her weapon behind or in front of her.
-Additional "Cheer" animation(s): Casablanca, Hinoki Cypress, Snowflake, all 3 Tulip sisters (and it's same pose too!)
-Snowflake and Violetta/Purple Tulip got their Battle Skill attack (Inhuman Slash and Tornado Cutter respectively) recolored (to from purple to green and green to purple, respectively.)
5 Stars
(Everything's scattered as far as who I have and who's evolved, so I'll just leave what I know.)
-Additional "Standby" animation(s): Cymbidium, Lily Rubellum (2), Convallaria (hard to notice), Violet, Bipinna Cosmos (3), Strawberry Candle (hard to notice),
-Additional "Move" animation(s):
-Additional "Cheer" animation(s): Setaria (OH MY GOD THAT IS ADORABLE), Rain Lily (?), Lily Rubellum, Scarlet Tulip
6 Stars
I only have Bride Oncidium, Victoria, Sakura, Maple, and Alstroe Maria. (The rest I've seen via helpers or the shop.)
-Slight sprite update for Black Baccara.
-Slight sprite update for Alpinia (the blades look shinier).
-Slight sprite update for Mei Plumblossom? (Her skirt looks darker to me.)
-Additional "Standby" animations: Black Baccara, Alpinia, Mei Plumblossom(2)(?), Cattleya, Dogwood, Lycoris(?), Acacia (2)(?), Wax Vine (2?), Burning Bush, Alstroe Maria.
-New "Move" animation for Ionpcidium (via helper).