powder company's go to lengths to blend powders to match and maintain certain set pressure curve parameters for each powder type/number
but as powder ages its going to change its characteristics very slowly, If the powder was properly stored (cool, Dark, & dry) and under 20 years old it should be stable and consistent,
Ive had few issues with properly stored powder, even 30 years old, but if its stored where its getting temp and moisture swings it will degrade faster