Want a romance where at least one of the love interests, or the main char, change age under certain conditions. I.e. bastard(MC) love hina(the tan chick)

I'd prefer if it happened somewhat frequently, as opposed to like 2 times like in love hina x.x. anyway aside from that anything other than hentai goes.

Few favs
Comedy:
Kyou kara ore wa!!, Mx0, Angel Densetsu, Skip Beat, Ai Kora, The Devil King is Bored

Romance:
Hana to Akuma, No bra, Shinigami Trilogy, kindan no koi de ikou, Usagi Drop, Threads of Time, Girl Friends

Most under rated:
Kindan no koi de ikou, kyou kara ore wa!.
m0r l83r... maybe >_>

Mahou Tsukai Kurohime - female lead/love interest is sealed in a child body... she transforms into her true adult form to fight with the help of the male lead (a kiss or something? i forget)
Dance in the Vampire Bund - love interest changes into adult form sometimes iirc...

Merupuri should work.

Unfortunately I haven't read the given examples, but maybe you can try Tenjou Tenge, because one of the main characters is (at least in the Anime) able to change into a childlike form, or The Bride of the Water God, if you don't mind Manhwa? ^^"

The storyline gets by time messy, because of the many twists and characters involved, but the male main character changes his form, too. By day he is cursed to look like a child (Habaek) with reduced powers and by night he gets back to his adultlike form (Mui). Also, the female main character is in the beginning unaware of those changes.

"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

Sakuranbo Syndrome
Kamisama Damono
Mayonaka Lolita
Dragon's Son Changsik - sadly most of it it's untranslated
Howl's Moving Castle (anime)
Kimama ni UpDown - I dropped it after a chapter or two, because it seemed like crap though
Magico - though the character who can do age transformation does it only twice... she isn't a love interest either

I can confirm that all the series mentioned by everyone else have age transformations too (except for Tenjou Tenge which I haven't read).

Kurohime, although it's not primarily romance

"People don't change, they just give in."Kubera is the best.同人音楽 (doujin music)

Mahou Sensei Negima!