Maintaining the integrity of URLs while moving pages between site sections in Drupal 7
If you are referring to the normal url, like node/3, then the url is always the same.

If you are referring to the path alias, which is the pretty url you can give to a page, or configure to generate automatically with tokens via the pathauto module, then when you move a node in to a different section the alias can change (this depends on how you have the alias generation configured, for example if there is nothing related to the section in the path alias then there will be no affect).

If your alias changes when you change a node, then normally the old path will no longer work and you can end up with people linking to those pages having their links break.

A workaround for this is to use the redirect module.

If you have pathauto and redirect both installed, then on the pathauto settings page you can configure the "Update action" option to create a redirect between the old and the new alias when a node is updated. This will stop incoming links from breaking when you change a node and its alias changes.

I would also recommend configuring the redirect module to delete old redirects if they haven't been used for a period of time (say 4 months for example).
This means if no one has used the old path in 4 months then it will delete the redirect as you likely don't need it anymore. Set the period to whatever you feel comfortable with.

I recommend not using this auto-redirect on alias change feature during development if content is likely to change a lot. This is because you end up with a lot of redirects that are not needed and if you change a node so that the alias changes from alias1 to alias2 and then back to alias1 again, you can get errors. If you care to learn more about those errors, see:


http://drupal.org/node/1817976
http://drupal.org/node/1263832
http://drupal.org/node/1796596