From 1948-58 there was no sergeant with three stripes. the grade structure went from Pvt, PFC, Corporal, Sergeant, Technical Sergeant and Master Sergeant and the grade structure went grade 7 (no chevron), grade 6 (no chevron) grade 5 (PFC), grade 4 (corporal), grade 3 (sergeant), grade 2 (technical sergeant) and grade 1 (master sergeant or first sergeant). A sergeant wore three chevrons and one rocker. When the new yellow on green chevrons were adopted in 1958 the rank of sergeant with three chevrons was reinstated and the grade structure was realigned.
Regulations allowed a soldier who was a Sergeant or a Technical Sergeant prior to 1958 to continue to wear the old style chevrons until promoted to a new grade. These soldiers were originally in the grades of grade 3 and grade 2. When the conversion happened these soldiers converted to E-5 and E-6 respectively. If a soldier had enough time in service and time in grade they were converted to E-6 and E-7 and wore the new chevrons for that grade.
Many NCOs wore the old style chevrons to show their longevity and so that they would not have to spoil uniforms by sewing on the new chevrons over the stitch marks of the old. That would have looked like they had been reduced in grade and rank at some point. That was a big morale issue at the time..