+# Implementation note:
+#
+# If you pkgpart-change a package that has been billed, and it's set to give
+# credit on package change, then this method gets called and then the new
+# package will have no last_bill date. Therefore the customer will be credited
+# only once (per billing period) even if there are multiple package changes.
+#
+# If you location-change a package that has been billed, this method will NOT
+# be called and the new package WILL have the last bill date of the old
+# package.
+#
+# If the new package is then canceled within the same billing cycle,
+# credit_remaining needs to run calc_remain on the OLD package to determine
+# the amount of unused time to credit.
+