The short answer is technically yes, the bank can change your locks. When you signed your closing documents you signed a security deed that likely allowed the bank to take care of the property in your absence. When the bank hires a contractor to verify if the property is occupied, he has two check boxes, occupied and ABANDONED. If it’s abandoned, then the bank will change the locks to “secure” the property.
What is baffling is the practice of changing the locks and then putting bold colored stickers to let everyone in the neighborhood know about this.
In addition to changing the locks, the bank can begin to “preserve” the property by cutting the grass and doing routine check ups. This is usually by a contractor (vendor) that knows there is no accountability and as long as no one complains TOO much, they can keep their contract.