How can I change the boundmode attribute of sample. I just want to do a simple zoom & offset effect and I want to be able to change dynamically the boundmode. I know it's pretty silly but I don't get it from the doc.
You can't. Although gen operators resemble standard Max externals in terms of letting you set initial states using attributes and arguments, you can't change them once they're set - they're strictly declarative. Have you noticed that typing "+" instead of "+ 5" gives you different looking gen operators? Same idea. That's all covered and described here: