While I don't think I can answer every component of the question, allow me to see what I can and I'm sure other members will also chime in as well.
- If you "cancel" your cell phone contract with a carrier they release your phone number, which will most likely occur in approximately 3-6 weeks. I had a cell phone number with Verizon Wireless and I cancelled out of my contract after 3 years, within about a month the number was reassigned to someone else (with Verizon). The same thing happened with a T-Mobile account I had for about 2-years, only they recycled it about two weeks after I cancelled it.
- You can "port" your number to another phone, or even another carrier but in order to perform that action, the account (with that number) must be active. I've heard of some people being able to port numbers up to 10-days after they cancelled their account, but I also heard that it was a nightmare to arrange that action. Not something I'd expect to be a regular exception made by carriers.
- Regarding the employee discount, you might wish to speak with your employer (as well as AT&T) to see what their agreement is for employees. Perhaps there's some line item in the Terms & Conditions that specifies that. Typically, there's no cut and dry when it comes to discounts for employees of companies that have agreements with carriers.
As for the overall scope of the dilemma, it certainly appears as though you need to speak with AT&T further to resolve the specifics of the issue. Especially regarding the ability to retain your current number. Good Luck!