Greetings Karan,
The Key Date that you supply when you are creating the Task List is the earliest date at which the Task List is availabkle and can be included in Orders or scheduled in Plans. Therefore, as Maria Terence mentioned, it is not possible to use, or even find it in selection, if a date such as a Maintenance Plan Schedule date is before the Key Date of the Task List creation.
It is a good practice to input an earlier date when you are creting Task Lists for PPM purposes, e.g. the start of the previous year, or the start-up date of the technical asset or installation.
If you have Task Lists for which you need to set an earlier Key Date than initially created, you may create a copy of the existing Task List (specifying the earlier Key Date), then delete the original Task List in IA25.