When does a forked project require a license?

Question

I haven't quite grasped the implications of forking a licensed project to public.

I am assuming that a similar license should be kept, but what happens if that's not possible?

Edit

I am providing an example (as suggested by Michael Schumacher in the comments):

Font Awesome is under the SIL OFL 1.1 license. If one is to fork it in order to extend or modify the font glyphs and thus modifying the font files, would the forked project be under the same license through a DerivativeRenaming clause? How should it be phrased in order to keep complying with this license?

Or, since the license seems to specifically refer the owner in its usage terms, would this usage still be appropriate?

Already forked projects, such as font-awesome-rails, refer to the exact same license, but I found none that actually altered the actual fonts, therein lies my question.

Answer 1

When creating a fork of a project, that fork will have the same copyright license as the original project.
Only the owner of the copyright can give away some of his rights by means of a copyright license. In an open source license, the common rights that are given away are the right to make copies, to make changes and to (re-)distribute either the original or the modified work.

One of the rights that a copyright owner has is the right to determine if a (modified) work can be distributed under a different license than the original one. Licenses that don't allow a covered work to be distributed under a different license are commonly called copyleft or ShareAlike licenses.
The SIL OFL 1.1 license has a ShareAlike clause, which means that if you fork a project with the SIL OFL 1.1 license, then your fork must use the license.

The 'DerivativeRenaming' clause in SIL OFL 1.1 means that if you change the appearance of the font, you must also call the font something differently. This is usually done to avoid confusion, so that users will know exactly what a letter A looks like if they talk about Font Awesome.

Answer 2

An Open Source license allows forking and redistributing with changes under the (at least) same license under which you got it (though there may be trademark issues). This is one of the fundamental requirements to be Open Source.

Using the same license is not necessarily required but depends on the license of the project.

A copyleft license like GPL requires that the same license needs to be used by the fork. Unless there is a clause (exception) that lets you release the fork under a different license.

A more permissive license could let you even close source the fork.