Mass schedule Planned Orders ?

Dear Seniors,

We have a bunch of Planned Orders created which have only Basic Dates and no Scheduled Dates.

The routings used at that time were dummy.

Please let me know if there is a way to schedule a bunch of Planned Orders in mass, backwards so that the Basic End Date is not changed, and the Basic Start can follow the scheduled date.

I have tried by using the MRP but it does not work for Firmed Planned Orders.

If I un firm them, then the dates will be totally changed. We want to keep the Basic End Date unchanged.

Thank you,

Alex