Creator Eric Kripke wants the show to continue, only not the same way it is now. He wants to get two new leads for the show and make it more of a light, music-oriented show, sort of High School Musical meets Rocky Horror Picture Show. OK, I made all of that up. But he does want to change the way the show is, so it can be renewed but be different, the way The X-Files and Buffy The Vampire Slayer changed their shows. He says that this upcoming season (season five in a long-planned five season arc) will "end with a bang."

http://ausiellofiles.ew.com/2009/08/30/will-supernatural-end-in-2010/

Whatever happens, I would hope that they keep Sam and Dean Winchester
as the main characters. For me, The X-Files went downhill fast when
they phased out first Fox Mulder and then Dana Scully. It just wasn't the
same show. I would hate to see the same fate happen to Supernatural.

Whatever happens, I would hope that they keep Sam and Dean Winchester
as the main characters. For me, The X-Files went downhill fast when
they phased out first Fox Mulder and then Dana Scully. It just wasn't the
same show. I would hate to see the same fate happen to Supernatural.
Sometimes it's better to go out on top. Look what's happening to Smallville.

If they change it too much or get rid of the two main characters that could really put a end to it.

Yes. Sam and Dean are great characters, they shouldn't be fdaded out and then be like replaced.