I'm looking for a manga where the main character (preferrably male) decides to change himself physically. Generally speaking, going from really unattractive to attractive. This change could have happened prior to when the manga takes place, but I'd prefer to see the change take place somewhere in the story, probably early on. Would also like it to be romance/slice of life, non-action if possible.

A manga I've read that fits the bill: Ane no Kekkon

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Cna yuo raed tihs? Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it dseno't mtaetr in waht oerdr the ltteres in a wrod are, the olny iproamtnt tihng is taht the frsit and lsat ltteer be in the rghit pclae. The rset can be a taotl mses and you can sitll raed it whotuit a pboerlm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. I awlyas tghuhot slpeling was ipmorantt!

Definitely try Lookism and let me know what you think!
Holyland has action which I know you're not looking for, but the character development for the MC is insane. It's my favorite manga, and I highly recommend it.