That's not entirely correct. A retired officer remains on the Army's rolls and may be recalled to the day he dies.
Similarly, all retired personnel, officer or enlisted who are eligible for retirement pay remain subject to the UCMJ, although this is rarely exercised.
An officer who has completed all his service obligations and tenders an unqualified resignation before retirement eligibility is not subject to recall.
Once you’re in the computer, you’re in the computer. You can resign your commission, but you’re still in the computer. An enlistment is a contract to serve for a given time. No problems unless the Chinese invade us, then, you’re in the computer.