Someone else you say?
Supposing you mean the author of the rest of the series, no.
J.K Rowling wrote Harry Potter 7!
Answer: No
If you are talking about Daniel Radcliffe yes he did play harry potter in the deathly hallows... :)
That would be against the author's rights, but there are quite a number of fan-fictions about these characters.
Harry Potter books? No. Harry Potter Fanfiction? Heck yes. Fanfiction is basically the fans' way to write stories about HP, and some of them are as long as books.
J.K. Rowling (author of the Harry Potter series) has stated that she may or may not write more books about Harry Potter or his offspring. Since then, she has not stated anything else.
Polyjuice potion transforms the drinker's appearance into that of someone else.
Thousands audtioned for the role of Harry Potter.
No. She said she's likely not going to write anything else Harry Potter related except perhaps an encyclopedia. She didn't rule it out entirely though.
From the books, where else?
you don't have to show your cards when someone else plays the Howler card.
Daniel Radcliffe was chosen for the role of Harry Potter. The movie producers had audtioned thousands of children for the role. They had almost given up hope when a producer saw Dan in the audience of a theatre production. He had the look so he asked him to try out.
No, Daniel Radcliffe will resume his role.
Why? For the same reason that anybody else eats.