Most wikis now use CC-BY-SA, including most of the Linux wikis I've looked at. CC-BY-SA is the more standard license for written content. I think it's a good idea to change - but actually the Wikimedia dual-license model is even better:
The dual-licensing option used by Wikimedia is probably the best option for a Linux wiki, where there is still some interest in GFDL. It's a little complex to explain - see http://en.wikipedia.org/wiki/Wikipedia:Licensing_update
In Wikimedia's case, this means that the end-user can choose either license, *except* where someone has added CC-BY-SA-only content to a page, which makes that page single licensed. It is the end-user's responsibility to check.
In theory GFDL could be chosen instead, so GFDL content could be added, but CC-BY-SA content could not be. As I've said, though, CC-BY-SA is the more common license, so I'd suggest this as the default.
So, my suggestion is to use the same variation of a dual-license as Wikimedia.
In case anyone is wondering, the operator of the wiki can make the decision to change license, until the end-of-July deadline. See http://www.gnu.org/copyleft/fdl.html#section11
for this and any other details.
If there's any uncertainty about this, with the deadline coming up for
the license transition (end of July), then there is a solution to give
more time: switch to a simple dual license. This means all content is licensed
under both GFDL and the CC-BY-SA licenses - but we can't add
material from other sources (if it's only under one of the licenses).
Then when a decision has been made to go with a particular option, it can be switched.
Let me know if anything is unclear.