Hi,
i want install easy profile in my site already online from many years and i have already registered users: any problem in that?
No problem, Easy Profile extends Joomla user management, so all Joomla users are also Easy Profile users.
You can run "Sync User" (button) into Easy Profile administration to create row in in DB into Easy Profile table, in most cases this is not necessary, It should be used if you encounter problems (but I think this is not necessary).
from many years
Keep in mind that Easy Profile is not compatible with Joomla 2.5 series.
user list module (or component?) will show user data fill in with easy profile?
In these list you will see all Joomla Users (also pre-existing users)
any problem with Payplans registration process? So user have to choose plan before complete the registration?
No, until 6 months ago, we used payplan in this site
Easy Profile extends Joomla User Management, so registration/edit profile page is the same of Joomla (custom fields are added by our user plugin and tabs by javascript), then workflow is the same with only Joomla registration.
We recommend you to try Free version before to understand how to work Easy Profile, in free version is available user list view (no module).