Just FYI...

When I went from active duty to FTS (TAR) my designator (USN) changed from 1310 to 1317. They are both active duty designators but I was dropped from Tricare and had to "resubmit" my paperwork. I took this way up the Tricare chain as I was never anything but active and my son was denied a sick call visit. Their response was anytime you change from the regular active designator it is assumed you got out...i.e. programmed in the software. Everyone of my buddies had to resubmit when going SELRES or FTS. My recommendation...

Track and push your status change in DEERS. The instant you show as reserve submit the forms with TRICARE. Get friendly with the DEERS and TRICARE office personnel beforehand to help you fast track the process. If I remember correctly there was still a time delay (few days) for the "re-enrollment" to take effect.

Good Luck