Did the alpha significantly decay between periods?

A strategy's alpha is estimated in two periods. In the early period it is $6.0$ bp with standard error $2.0$ bp ($z \approx 3.0$, significant). In the later period it is $2.0$ bp with standard error $1.5$ bp ($z \approx 1.33$, not significant). A portfolio manager concludes the alpha has "significantly decayed."

Test whether the change in alpha between periods is statistically significant, and explain why the two separate p-values do not establish decay.