The engine developers are looking to drop support for the 32-bit builds so, after some testing and profiling, this update applies the 64-bit engine to everyone with a 64-bit system. Previous attempts to do this hit bugs and performance issues, but none of those were found this time around. The 64-bit build appears to be faster and able to run larger and longer games without apparent issue. Be sure to report any if you find them.
In other news, 32-bit Linux is no longer supported.