Similar to WordPress, WP-Members has pluggable functions. This allows you to replace a function in the plugin without modifying the plugin’s core code, a bad practice that makes upgrading difficult. Using pluggable functions for customizations makes upgrading easier, since your custom functions are maintained outside of the plugin’s files and are not overwritten when you upgrade.
It should be noted that even using pluggable functions, you can break the plugin. You should pay careful attention to the original function(s) and what they are doing. Also, when you upgrade, you should review the upgrade’s changes/updates and checked your custom functions to see if there are any compatibility issues to be addressed. If there have been changes to a function you are maintaining a custom version of, you should carefully review to see if you need to update your customization.
Here is list of pluggable functions found in the plugin:
wpmem_inc_registration_NEW (to be deprecated)
wpmem_login_form_NEW (to be deprecated)