Hi. 

I saw this Drama a long time ago, that's why I don't remember so much anymore. 

But I am pretty sure, it was chinese. If it wasn't, I'm sorry.

Plot:

A hardworking Girl has a big red birthmark on her face. She is nice and kind and helps people a lot. But she feels ugly

because of her birthmark. She has a beautiful Sister, and one day, magically, she switches the body with her sister

and is beautiful for the first time in her life. 

Can anyone remember a drama like this?

I mean... I don't think this is it but there was a Hong Kong drama called Queen of Diamonds & Hearts.

She has a huge red birthmark beside her right eye. She becomes a queen. She switches bodies with her "sister queen" (not her actual sister - polygamy).

But the reasons I'm doubtful is because the plot is focused on the incompetent king fawning after his beautiful queen while neglecting his other queen (the one with the birthmark who is working to help/save his empire). Now there is a bodyswap but it happens much later on in the show and it's just kind of thrown in. 
That sounds like King Flower, also known as Substitute Princess. I ended up dropping it, so I don't know if they ended up being sisters, but they were similar and when the rich pretty one fell off a mountain and went to the hospital, the ugly poor girl got plastic surgery to make her identical and pretty and take the other's place so her family wouldn't find out. It is a Taiwanese drama.