Is it possible to change woocommerce plugin by wp ecommerce?

Yes it is possible. The plugin is open-source so you cna make any changes that you want to.

Just remember that any changes that you make will be over-written by any future updates, so you’ll have to remember to never update again.

If that doesn’t suit you, I know that Woocommeerce has a wide range of filters and actions set up, so you can use a lot of those to make changes to the way that it works.