Technically your mortgage or note probably has an occupancy clause tied to it, however these clauses are not air-tight and extenuating circumstances like the family out growing the home would be a good reason to move. Odds are so long as the mortgage continues to be made on time you will never have an issue.
If you apply for a new loan to purchase a home FROM THE SAME LENDER, they may give you a tough time because you represented to them that you'd occupy the old home. But if that's the case then just get approved through a new lender.
If you'd like a new pre-approval feel free to give me a call and I can walk you through the process and help identify your options and any roadblocks. I'm local, experienced, and can even refer you to a Realtor if you need one