best practice in managing different versions of java and web project

Hi
Suppose if i start with a java project and have 500
java programs, and then i release this project and
name is version 1.0
Now i have to add some features, in next version which
is 1.1, 
so what do i do copy all the 500 programs in version
1.1 and then add features or keep the 500 programs in
1.0 version and add only new programs in version 1.1
and then use tool like ant to build new project
version 1.1 for release?
This will be the way to do in RPG, since u can control
which program to be used by simple library list, but
it is complicated in java due to classloader etc.
same in case of jsps and xml files and web projects
so what do people do when they have to work with
different versions of java project and relese one
after another, and also fix bugs and support the older
versions??
can anyone explain how they do it in big projects or
open source projects
Ashish


                
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