Changing AAM already defined when the farm and web collection was created

Hi   I am fairy new to SharePoint and inherited a farm built previously by a project manager not an Admin, i am also not a SharePoint admin but been asked to manage the server and gain the necessary skills.

the AAM default was left with the server name which seems fine for me.

my problem is that the project manager created a http://company.sharepoint   internet zone  http://company.sharepoint

and this is the URL all internal company users use, when they open documents they are prompted  for a password, info path forms have an error about submitting forms in cross domains, people picker has foundation error when adding a user with the Browse  icon. everything works when i use the default URL which makes me believe that the URL he created above should had been associated with the intranet zone ? that why office prompts for a password the fist time on a session, and info path thinks this is another domain?  can i just change the zone in the drop down to intranet whiteout causing any other problems ?  there are no external users and no internet users, only our domain and our sister company which already has two way trust and direct connection to our network. i just don't want to take a change of braking it until i learn more about

thanks