Change Base Unit of Measure from PCS to GMS

Hi

I am using Nav 2009 R2 and find below an example of item I have in the item card:

ITEM NAME : Dates 500gm Base Unit of Measure : PCS

However now I could like to change the Base Unit of Measure from PCS to GMS

When I try to change it shows the below error: You cannot change Base of unit Measure because there are one or more open ledger entries for this item.

How can I change the Base Unit of Measure? Kindly help

Thanks