The rules talk about financial hardships that were not foreseeable at the time you bought the house, and there are several "safe harbors" -- if you fit a safe harbor situation, you qualify for the partial exclusion. (If you don't fit the safe harbor, you can still qualify depending on facts and circumstances.)
A job relocation that changes your commute by more than 50 miles qualifies as a safe harbor, it does not have to be forced. But note that what counts is the change in length of the commute. If you currently commute 20 miles to the east of your home, and your new location is 40 miles to the west of your home, that does not qualify because your commute is not 50 miles longer, even though the jobs are more than 50 miles apart.
You may still have to pay some capital gains tax if the gain is larger than the partial exclusion. If you are single, and lived in the home for 16 months, your exclusion is $250,000 x (16÷24) or $166,666. Also note the exclusion is based on the shortest of 3 time periods: how long you owned the home, how long you lived there as your main home, and how long since you last used the exclusion. Therefore, if you move right away for the job and it takes 3 months to sell the house, your exclusion is based on 16 months, not 19 months.
