I think you're talking about the WAN IP (the one MTS gives you). The IP isn't static, although it's "sticky". Essentially, you'll generally keep the same IP as long as your 2Wire RG isn't having network problems or is unplugged for an extended period of time (more then 48 hours). It's also possible it can change if MTS changes their network configuration or does maintence.
The short answer to changing it on your own is that it's not really possible, unless you're willing to leave your modem unplugged for awhile.
When it comes to security, which I think is your concern? The 2Wire RG has a built in firewall that is enabled by default. If you wanted additional security you could also run a software firewall on your home PC. Changing WAN IP addresses, does not really increase the security of your PC.
If you're concerned about security, I recommend the following:
1 - Ensure Windows Update is enabled and runs often
2 - Ensure Adobe Reader, Flash and Java are up-to-date
3 - Use Internet Explorer 8, Mozilla Firefox or Google Chrome
4 - Use a hardware firewall (IE router, included with Ultimate TV) or a software firewall in place of, or in conjuction with
5 - Use Microsoft Security Essentials (free anti-virus/malware) and ensure it is up to date
6 - Ensure you use secure passwords (at least 8 characters, consisting of numbers, letters and special characters - and not containing any words that can be found in a dictionary). Passwords should never contain or be the same as the username.
7 - Do not use the same password for everything (your facebook account, should not have the same password as your e-mail and online banking).