The defaults for the x264 CLI (command-line) encoder have changed in r1177 indeed. But that doesn't effect Avidemux at all. It has its own default values
But I agree that the x264 dialog in Avidemux needs some updates:
* SubME=10 ("QPRD") is missing and only available in Avidemux through a patched libx264
* Psy-RDO and Psy-Trellis cannot be adjusted, because options are missing
* AQ mode 2 (aka "AutoVAQ") is missing too and again only available via patched libx264
So adjusting Avidemux' x264 dialog for the new x264 defaults is definitely
not the most important thing to do.
You can always choose your preferred settings. Don't worry too much about the defaults...