Why? Just replace the old toolset with the new.
Erm... that's exactly what I'm doing... I even said so
... replacing the 4.3.2 avr-gcc toolchain files with Atmel's version that supports the chip.
I see where the confusion originated. Arduino, of course, comes with the toolchain, so I was using the term "Arduino IDE" to refer to all the crap you get when you install Arduino... java app (processing), toolchain, etc.
I wished it was a simple matter of tar zxvf but instead, the directory structure for Arduino avr-gcc (on Linux, at least) and a standard avr-gcc install aren't the same. For example, libexec is where cc1, cc1plus, and collect2 live in a standard avr-gcc. Well, they live in lib within hardware/tools/avr/lib/ within Arduino (1.0.5 is what I happen to be working with).
Also, avr-g++, avr-gcc, and avr-cpp are scripts that call files out of hardware/tools/avr/bin.gcc so that LD_LIBRARY_PATH can be set. Considerably more pain comes with the lib directories. You've got to find and deal with libgmp, libmpfr in different spots, libgcc, libgcov, in different spots, startup object code in different spots, and on and on. And then there's linker scripts in one place versus another, etc. I got to the point of linking, and had to quit last night.
If only I'd had a chance to poke around inside gcc more in the last 25 years.
It's always just been ./configure && make install and it's done after awhile (actually it was more painful and the compile took ages longer back in the day on old Dynix/PTX systems, Solaris 2.x, or even the club's salvaged VAX-11/750 running 4.3BSD... but I digress).