define a number field for your primary key. Use field options to specify serial numbers.
Put this new field on a layout.
Show All records
Put the cursor in the field.
Select Replace Field Contants from the Records menu and choose the Serial numbers option.
Click the option for updating entry options also
Perform the replace.
If you are using FileMaker 12, you might use Get ( UUID ) as an auto-entered calculation on a text field instead of a serial number field for your primary key. Replace Field Contents can be used to give such a value to all existing records or you can use this trick: