The external URL is right, but the internal one depends on your current configuration. Do you have split DNS, legacy Exchange servers etc?
Here is a snapshot of the authentication methods:

"is there any possibility of ANYTHING that's currently working being effected by the above Command?"
All you clients that are currently using EWS, like:
- Entourage with EWS extension
- Outlook 2011
- Apple Mail
- BlackBerry Servers: Mozilla/4.0+
- Microsoft OCS 2007 R2: OC/3.5.*.*
- Microsoft Lync 2010: OC/4.*.*.*
- Microsoft Lync Web App: CWA/4.*.*.*
- Microsoft Lync Phone Edition: OCPhone/4.*.*.*
None of the above is used - only Outlook 2007, Outlook 2010 and Outlook 2013
Webaccess, iPhones, Android Phones via Activesync
The reason for asking is do I risk the chance Breaking existing functionality... (company is very busy at the moment & can't risk breaking anything)
I was thinking of changing the URL via ECP (I think that is where it was first setup!)
I guess a REBOOT will be necessary....
if still not working - I can change it back!
NB: I used the procedure of setting Internal & External URL the same as per deployment assistant
http://technet.microsoft.com/en-US/exdeploy2013/Checklist?state=2284-W-DwBEIgAAQAAgAAEAAQAAAA~~