How to turn off amd64?
 

I have installed Debian Lenny from the distribution and have choosed "amd64" version.

What does it mean? It concerns only kernel or everything?

Having this system installed, can I turn amd64 back off without reinstalling it? Is kernell recompilation is sufficient?

It is the 64 bit version of the distro. If you have an x86_64 chip, it is OK. You may have an option in your package manager to install 32 bit versions of applications if your distro is biarch. Some apps are 32bit and 64 bit versions aren't available. One example is Open Office AFAIK.

It means you have a 64-bit kernel that can support 64-bit applications. ("AMD64" does not mean it is only for AMD CPUs. "AMD64" is the same as "X86-64" and is for most Intel 64 bit CPUs as well).

It tends to mean everything, not just the kernel.

You can run 32-bit applications under a 64-bit kernel. But installing them may be tricky. Since you have a 64-bit kernel, the package management software (normal method of installing applications) will choose the 64-bit version whenever it is available (which is almost always).

No. To switch from 64-bit to 32-bit, you would need to reinstall the whole distribution, not just the kernel.