I wondered what happened to the Grunts and the rest of the HL:Alyx human combine in Half Life 2. Its either they're converted into other units as they were discontinued, shipped offworld, or remain as other smaller subunits. I decided that It would be cool to blend some Beta ideas, and that the Grunts are downgraded or converted to Civil Protection Elites as Squad Leader. And probably Wallhammer, APF, and Ordinals are converted to Soldiers or Elites Soldiers. Or maybe they remain, but as they die they dont get replaced by new Grunts or Ordinals or the rest of the HL:Alyx units, but by the Soldiers and Civil Protection units we know in HL2
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Well, according to HLA models and lore - all those fellas may have been promoted to Overwatch soldier or else with better equipment, knowledge and responsibilities, since they have implants installed and can't be in Civil Protection. Their "finest" minds can be saved in the Combine's storage (EZ2 moment) and, by the right time, they can be transplanted into new bodies to do their tasks. And "old" bodies can just be: buried, disassembled, destroyed or else. Why not. Since the "NEW TIER" - Civil Protection units can be promoted to Overwatch soldier & Elite only.
It is possible they're either removed or still exist. The HLA units were there to guard specifically the Vault and are usually found in the QZ, while what we see in HL2 were in either the Outlands or were the standard city cops. Judging how the Vault was destroyed, they're either gone or were assigned to patrolling in just the QZ.