You can use a script in the Transform node.
You will need to store the current value as a variable so that it is available when the node processes the next record:

ConfigureFields Script:
## Define a field to store the calculated value
out1.result = int
#Configure all fields from input 'in1' to be mapped
#to the corresponding fields on the output 'out1'
out1 += in1
ProcessRecords Script:
#Copy all fields from input 'in1' to the corresponding output fields
#in output 'out1'. Copies the values of the fields that have been setup
#in the mapping defined in the ConfigureFields property
out1 += in1
if node.firstExec:
previousValue = 0
## If the current value is Null use 0 instead
if in1.rand == Null:
currentValue = 0
else:
currentValue = in1.rand
## Output the calculated value
out1.result = previousValue - currentValue
## Store the current value for use with the next record
previousValue = currentValue