Maintain History of KPI when switching between Enhanced SLA's

Hi Guys,

I have 2 Enhanced SLA's - First Level Escalation and Second Level Escalation. 

The requirement is that I need to maintain the history of KPI's when changing the SLA's.  So, for example, if SLA - First Level SLA was executing and 2 KPI's were fulfilled and then the SLA was changed to Second Level Escalation. Then, I need to show the earlier KPI's on the case too.