Ask HN: What are the rules around changing licenses of forked project?

Legally, you of course need to respect the license of source you forked from, so going from BSD-2 to GPL-2 you need to keep the authorship and license information from the original intact, because 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.. You couldn't relicense GPL code to BSD, because GPL includes restrictions not in BSD. Search keyword is "license compatibility".

Socially, there's the aspect that upstream typically can't make use of improvements to the fork without the contributors explicitly making it available under the upstream license too, which can cause additional friction.

Just outright replacing the headers is illegal. The original copyright owners have licensed the content under the specific terms of the BSD-2 license. Removing the BSD-2 header violates that license. Removing the copyright notice (which the BSD-2 header contains, unlike some other license headers) also violates copyright law.