Create python script to update field based on change in another field ArcGIS

I don't know what your environment is nor how you are querying/updating the table, but conceptually, I'd store a copy of the previous value that I can use to check against in the next loop. Perhaps something like:

oldValue = 0

[code to query current value]

if not currentValue == oldValue:
    TPN = newValue
oldValue = currentValue

Try this out. The SQL query may need to be changed depending on your circumstances. I gave this a spin on a few small feature classes and seemed to do the job.

import arcpy

fc = "X:\...\feature_class"

# features may or may not be sorted in attribute table;
# append each value of PASS_NUM to list and sort
print "collecting pass_num values"
pass_numbers = []
for row in arcpy.da.SearchCursor(fc, "PASS_NUM"):
    if row[0] not in pass_numbers:
        pass_numbers.append(row[0])
pass_numbers.sort()
print "found %s values"%str(len(pass_numbers))

# TPN_value starts @ 1 and increases +1 with each pass
print "updating values"
TPN_value = 1
for number in pass_numbers:
    rows = arcpy.da.UpdateCursor(fc, ["TPN"], "PASS_NUM = %s"%str(number))
    for row in rows:
        row[0] = TPN_value
        rows.updateRow(row)
    del rows
    TPN_value += 1

print "finished"