In order for a year to count towards the 20 year retirement a Soldier must earn 50 points during their "Retirement Year".
It is DoD policy that every person who enters military service by enlistment or appointment incurs a Military Service Obligation (MSO) of 8 years from that entry date, in accordance with section 651 of Title 10, United States Code. There are a few exceptions to this policy.
Officers can directly transfer from National Guard to Army Reserve. Enlisted Soldiers may be able to transfer as well depending on individual state policy.