diff --git a/mercurial/upgrade.py b/mercurial/upgrade.py
--- a/mercurial/upgrade.py
+++ b/mercurial/upgrade.py
@@ -152,7 +152,7 @@
 
 
 DEFICIENCY = b'deficiency'
-optimisation = b'optimization'
+OPTIMISATION = b'optimization'
 
 
 class improvement(object):
@@ -165,14 +165,14 @@
        will be mapped to an action later in the upgrade process.
 
     type
-       Either ``DEFICIENCY`` or ``optimisation``. A deficiency is an obvious
+       Either ``DEFICIENCY`` or ``OPTIMISATION``. A deficiency is an obvious
        problem. An optimization is an action (sometimes optional) that
        can be taken to further improve the state of the repository.
 
     description
        Message intended for humans explaining the improvement in more detail,
        including the implications of it. For ``DEFICIENCY`` types, should be
-       worded in the present tense. For ``optimisation`` types, should be
+       worded in the present tense. For ``OPTIMISATION`` types, should be
        worded in the future tense.
 
     upgrademessage
@@ -551,7 +551,7 @@
     optimizations.append(
         improvement(
             name=b're-delta-parent',
-            type=optimisation,
+            type=OPTIMISATION,
             description=_(
                 b'deltas within internal storage will be recalculated to '
                 b'choose an optimal base revision where this was not '
@@ -571,7 +571,7 @@
     optimizations.append(
         improvement(
             name=b're-delta-multibase',
-            type=optimisation,
+            type=OPTIMISATION,
             description=_(
                 b'deltas within internal storage will be recalculated '
                 b'against multiple base revision and the smallest '
@@ -595,7 +595,7 @@
     optimizations.append(
         improvement(
             name=b're-delta-all',
-            type=optimisation,
+            type=OPTIMISATION,
             description=_(
                 b'deltas within internal storage will always be '
                 b'recalculated without reusing prior deltas; this will '
@@ -613,7 +613,7 @@
     optimizations.append(
         improvement(
             name=b're-delta-fulladd',
-            type=optimisation,
+            type=OPTIMISATION,
             description=_(
                 b'every revision will be re-added as if it was new '
                 b'content. It will go through the full storage '
@@ -1297,7 +1297,7 @@
         ui.write(b'\n')
 
     def printoptimisations():
-        optimisations = [a for a in actions if a.type == optimisation]
+        optimisations = [a for a in actions if a.type == OPTIMISATION]
         optimisations.sort(key=lambda a: a.name)
         if optimisations:
             ui.write(_(b'optimisations: '))