Service members must be interchangeable. War is an industrial process. Only individual training and skill credentials matter. The only costs of moving a Service member from one unit to another are the financial costs of the move and the administrative paperwork. Like-credentialed troops are fungible.
Turbulence is a fact of life.
There s no way to replace casualties, except individual by individual. Units should be kept up to strength on the battlefield by the constant flow of individual replacements. Too much cohesion in small units is dangerous to the chain of command. Among units of like kind, uniform readiness and uniform capability are best. Weapons matter more than skill.