No impact if your MIC's validity is later than that of inspection plan.
Many organisations, infact, follow such practice where MICs are created time to time and they are assigned to an existing inspection plan.
Scenarios can be taken a laboratory test where chemical composition for normal steel is being analysed. Inspection plan would be having MICs for different elements
Organisation decided to manufacture stainless steel now and elements like Chromium, Nickel come into major play. Thus two more MICs for Cr and Ni would be created and they'd be assigned to same inspection plan.
However, when you assign new MICs to an existing plan then they'll appear for result recording only for thereafter created inspection lots. Previous inspection lot would remain unchanged.
NJ