You say you have an "internal ip" which suggests an ip assigned by a router on an internal network. Changing this ip will have zero effect on your external ip assigned by your isp.
Which you obviously know.....
So assuming your router is also your modem, and rebooting it hasn't worked.
Have you tried calling your isp and talking to them? tech guys are the same everywhere I'm sure if you get the right guy he'll happily discuss there systems and how they can be manipulated into assigning a new ip.
In order for the mac address trick to work
(assuming your isp is setup this way) you'd have to use a different modem or hack the mac address on your modem.... an easy way around would just be having 2 modems, pre-configured with your isp credentials and internal network crap, then switch as required.
lol bloody mish installing a new os just to change mac... you know you can use
"madmacs" on windows for mac changin.