If you have a separate home partition it is easy which is why it is a good
idea to set up this way. All you need to do is to re-install Ubuntu,
substituting 32 bit for the 64-bit one, BUT do NOTformat your home
partition. Just re-use it with the same user name.

Chances are that you are not set up this way because you need to do a custom
installation and most don't bother or know how. If you used the default
installation, your home partition will reside on the same partition as
Ubuntu and that means you will lose everything when you re-install ---
UNLESS you backup your home partition to a separate drive or device and
recopy it back once Ubuntu is re-installed. Make sure that you copy the full
/home folder including user name or if you only do the contents, make sure
that you have the hidden files, too.

I am curious why you are switching. I use 64-bit Ubuntu without any
problems, as do many others.