You can always relicence as long as the new licence is compatible with the old one.
No licence is GPL-compatible and will ever be so if you want to change the licence of a GPL code, you have to override the licence, wich can only be done by the copyright's holder
If the code is for example under unrevised BSD (with the publicity clause), you can't change the licence to GPL without the copyright's holder's consent.
If it's under MIT, new BSD, old X11 (original X11 NOT the new XF86), etc. then the code may be directy published under GPL since those licence do not add extra restrictions and do not forbid anyway the use of GPL.
XF86 was under X11-like licences so it was possible to add a restriction.
If your code is under GPL only the copyright holder can change the licence
WARNING: author != copyright's holder if you have transfered your right to another entity (FSF, University of Berkley, for example). Some projects do require this to accept your contribution)
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You can always relicence as long as the new licence is compatible with the old one. No licence is GPL-compatible and will ever be so if you want to change the licence of a GPL code, you have to override the licence, wich can only be done by the copyright's holder
If the code is for example under unrevised BSD (with the publicity clause), you can't change the licence to GPL without the copyright's holder's consent.
If it's under MIT, new BSD, old X11 (original X11 NOT the new XF86), etc. then the code may be directy published under GPL since those licence do not add extra restrictions and do not forbid anyway the use of GPL.
XF86 was under X11-like licences so it was possible to add a restriction. If your code is under GPL only the copyright holder can change the licence
WARNING: author != copyright's holder if you have transfered your right to another entity (FSF, University of Berkley, for example). Some projects do require this to accept your contribution)