A long time ago I applied to West Point from a college and also received ROTC nomination. I can't remember when I told them of my intention to reapply but I am pretty sure it was late fall. My cadre was very supportive. I believe most ROTC cadre will be very supportive to SA application. A ROTC PMS told his cadet that he will convince the cadet that the ROTC program is much better and she will end up staying. She reapply and went to West Point.
Too many consideration, but I believe early late October/early November will the right time to inform the ROTC cadre about one's desire to reapply to SA
First, should give some time to determine if the college and ROTC are meeting your need (i.e. like the college life, ROTC instruction is good or bad, etc). As ultimately, the goal should becoming an Army officer, not which school to graduate from.
Second, have to prove yourself in the ROTC program - by participating in optional activities and/or volunteering/taking on available challenging assignments.
Lastly, I believe SAs won't fully consider college applicants without their first semester transcript, so getting a ROTC nomination won't really change anything. ROTC nomination is not limited so the PMS can't say I used up my allocation.