MM! (AP potion)
 

This one is from an ecchi anime series called "MM!". A science geek girl with an undeveloped body for her age decides to take an aging potion in order to appeal to the main male character. Even though the AP is shown through her silhouette, the process is quite sexy. Not to mention that her voice contributes a lot as well. Sadly though, her AR sequence back to normal was not shown.

(17:38-18:12)
http://www.animeultima.tv/mm-episode-7/

P.S.
Since it's an ecchi anime, I was hoping that its BD would have the transformation uncensored. However, there was no change at all.

Re: MM! (AP potion)
 

things get very silly after the ap, but part of its charm I suppose

Re: MM! (AP potion)
 

It's like they combined Scott Pilgrim and Dragonball Z then smashed it into a stickyball and rolled it through the harem anime TVTropes page and used whatever got stuck.

Re: MM! (AP potion)
 

I remember when this got posted a while back originally. Thanks for the repost, it's a fun clip to watch anytime.

Also,

(goes super-saiyan)
LOLI-LOLI-LOLI-LOLI-LOLI-LOLI!

Re: MM! (AP potion)
 

LOL. Come to think of it, this episode teaches us that AP fans are the archenemies of lolicons!

Protecting every AP in the world, MAXIMUM EMU EMU!!!!!