I am going to fork an open source project. Would you recommend to create a new changelog file or to update the original one? Would it be reasonable to increment its version number or would it be better to reset it?
There's a lot to be said for resetting the version number to major+1.0.0, although some folks prefer 1.0.0 (which seems seriously wrong). Most importantly, it says "this is based on the major.*something*.somethingelse level of the forked package, but from here forward it's different". For example, Secure Sockets Layer (SSL) had 1.0 and 2.0, and the replacement Transport Layer Security (TLS) started at 3.0.
And yes, reset the changelog.
If by fork, you mean start an upstream incompatible project, then I'd recommend resetting the version number and simply referencing the old changelog on the first commit for your changes.
(Github and other dvcs services have added new meaning to "fork").