I have the folowing scenario:

I login. This starts BASH, with the proper environment; let's call this enviroment MAIN ENVIRONMENT; it has the folowing variables definiton: x=x_main; y=y_main. Now i issue startx. Everithing it's fine, works as i expect. Now i start xterm for example. From this, I want to change a environment variable from MAIN ENVIRONMENT, without closing a sinle window, nor X. Is here any way to do this?

The problem is that i want to cahnge the enviroment variables, and the changes to be reflected in every new started application, this without loging out, closing a windows or something...

Each process has a copy of the environment, so you can't change variable values of the parent's and siblings' processes after the fact. You can use the .profile or .bash_login scripts to change variables in the future, or alter them where they are defined (such as in a script in /etc/sysconfig).

Some variables are re-evaluated on the fly using functions, and for other variables, you may need to restart a service so they can be loaded in again.

One example, is HOSTNAME. While there is a "hostname" command that will reflect a recent change in your configuration, KDE will not work properly until you log out and back in again.