What is the best solution to replace a new memory allocator in an existing code?

From the TCMalloc documentation:

To use TCmalloc, just link tcmalloc into your application via the "-ltcmalloc" linker flag. You can use tcmalloc in applications you didn't compile yourself, by using LD_PRELOAD:

$ LD_PRELOAD="/usr/lib/libtcmalloc.so"

ptmalloc seems to be similar (but if you're on Linux, you're likely already using it because it's part of the GNU C library).

I would expect operator new to call malloc, but you can easily check for yourself by setting a breakpoint on malloc, then calling new. If your new doesn't call malloc, you can redefine it so that it does.