ладно,
Код:
#
# newbyte.py filename oldbyte newbyte [newfilename]
#

import sys

if len(sys.argv) > 3:
	namebin = sys.argv[1]

	if len(sys.argv) > 4:
		nameout = sys.argv[4]
	else:
		nameout = namebin+'_'

	b1 = int(sys.argv[2])
	b2 = int(sys.argv[3])

	with open(namebin, 'rb') as fbin:
		a = bytearray(fbin.read())
		a2 = []
		i = 0
		while i<len(a):
			if a[i]==b1:
				a2 += [b2]
			else:
				a2 += [a[i]]
			i += 1

		with open(nameout, 'wb') as fout:
			fout.write(bytes(a2))