Are you asking about a Planned Redispatch or some other reason for an enroute reclearance (e.g., weather diversion)?
In a Planned Redispatch, ATC is [usually] blissfully unaware that you have changed anything, since the original flight plan is filed as if the planned redispatch will be executed. The redispatch procedure is strictly between the company Dispatcher and the Captain on the airplane.
If you get an enroute reclearance for other reasons, the destination and alternate may be anywhere, depending on the circumstances...