It is not too late to change to another compatible license. More to the point, it is possible to use SE content with a compatible license even if the license notice on the site does not change.
Later versions of CC-BY-SA
Starting with the release of the 2.x license suites, CC expanded compatibility by allowing contributions to adapted material to be created under the same or later version of the original license, including other ported versions of the same or later version of the license.
(from Compatibility mechanism in BY-SA licenses on the CC wiki)
In other words, despite the 2.5 / 3.0 ambiguity, all the SE content could be:
- explicitly relabelled on the site as CC-BY-SA 4.0 (or later)
- used under CC-BY-SA 4.0 (or later) without explicit relabelling
Other compatible licenses designated by CC
The 3.0 Attribution-ShareAlike goes one step further, by allowing those contributions to be licensed under under a “Creative Commons Compatible License,” defined to mean licenses approved by CC as essentially equivalent to the 3.0 Attribution-ShareAlike license.
In other words, under CC-BY-SA 3.0 or later (which applies even to the 2.5 content, see above), all SE content could be used under a CC-approved compatible license without explicit relicensing.
The caveat to this is that no compatible licenses have been defined (yet).
For example, FDL 1.3 or CC-BY-NC / ND. No, changing would not be practical. See Mark's answer for details.
(Note that FDL may become compatible in the future. See Why does Stack Exchange not dual-license content under FDL? )