diff --git a/setup/migrate.py b/setup/migrate.py
index 3f581a82..9826d10f 100755
--- a/setup/migrate.py
+++ b/setup/migrate.py
@@ -197,55 +197,74 @@ def migration_15(env):
 		import yaml
 
 		with open(backup_configuration_file, 'r') as f:
-			custom_config = yaml.load(f, Loader=yaml.SafeLoader)
-		if not isinstance(custom_config, dict): raise ValueError()
+			old_configuration = yaml.load(f, Loader=yaml.SafeLoader)
 
 		# Converting the previous configuration (which was not very clear)
 		# into the new configuration format which also provides
 		# a "type" attribute to distinguish the type of backup.
-		scheme = custom_config["target"].split(":")[0]
-		if scheme == "off":
-			custom_config = {
-				"type": "off"
-			}
-		elif scheme == "file":
-			custom_config = {
-				"type": "local",
-				"min_age_in_days": custom_config["min_age_in_days"]
-			}
-		elif scheme == "rsync":
-			custom_config = {
-				"type": "rsync",
-				"target_url": custom_config["target"],
-				"min_age_in_days": custom_config["min_age_in_days"]
-			}
-		elif scheme == "s3":
-			import urllib.parse
-			url = urllib.parse.urlparse(custom_config["target"])
-			target_url = url.scheme + ":/" + url.path
-			s3_endpoint_url = "https://" + url.netloc
-			s3_access_key_id = custom_config["target_user"]
-			s3_secret_access_key = custom_config["target_pass"]
-			custom_config = {
-				"type": "s3",
-				"target_url": target_url,
-				"s3_endpoint_url": s3_endpoint_url,
-				"s3_access_key_id": custom_config["target_user"],
-				"s3_secret_access_key": custom_config["target_pass"],
-				"min_age_in_days": custom_config["min_age_in_days"]
-			}
-		elif scheme == "b2":
-			custom_config = {
-				"type": "b2",
-				"target_url": custom_config["target"],
-				"min_age_in_days": custom_config["min_age_in_days"]
-			}
+		conversion_succeded = True
+		if not isinstance(old_configuration, dict):
+			conversion_succeded = False
+			print("Exception when converting to the new configuration format: the previous configuration has an unexpected format.")
+		elif "type" in old_configuration:
+			print("The previous configuration appears to have already been converted.")
+			return
 		else:
-			raise ValueError("Unexpected scheme during the conversion of the previous config to the new format.")
+			try:
+				scheme = old_configuration["target"].split(":")[0]
+				if scheme == "off":
+					new_configuration = {
+						"type": "off"
+					}
+				elif scheme == "file":
+					new_configuration = {
+						"type": "local",
+						"min_age_in_days": old_configuration["min_age_in_days"]
+					}
+				elif scheme == "rsync":
+					new_configuration = {
+						"type": "rsync",
+						"target_url": old_configuration["target"],
+						"min_age_in_days": old_configuration["min_age_in_days"]
+					}
+				elif scheme == "s3":
+					import urllib.parse
+					url = urllib.parse.urlparse(old_configuration["target"])
+					target_url = url.scheme + ":/" + url.path
+					s3_endpoint_url = "https://" + url.netloc
+					new_configuration = {
+						"type": "s3",
+						"target_url": target_url,
+						"s3_endpoint_url": s3_endpoint_url,
+						"s3_access_key_id": old_configuration["target_user"],
+						"s3_secret_access_key": old_configuration["target_pass"],
+						"min_age_in_days": old_configuration["min_age_in_days"]
+					}
+				elif scheme == "b2":
+					new_configuration = {
+						"type": "b2",
+						"target_url": old_configuration["target"],
+						"min_age_in_days": old_configuration["min_age_in_days"]
+					}
+				else:
+					conversion_succeded = False
+					print("Exception when converting to the new configuration format: the previous configuration has an unexpected scheme.")
+			except KeyError:
+				conversion_succeded = False
+				print("Exception when converting to the new configuration format: the previous configuration is missing one or more keys.")
+			except:
+				conversion_succeded = False
+				print("Exception when converting to the new configuration format: generic exception.")
 
-
-		with open(backup_configuration_file, "w") as f:
-			f.write(yaml.dump(custom_config))
+		if conversion_succeded:
+			with open(backup_configuration_file, "w") as f:
+				f.write(yaml.dump(new_configuration))
+		else:
+			with open(backup_configuration_file, "r") as f:
+				lines = list(map(lambda line:"# " + line, f.readlines()))
+			with open(backup_configuration_file, "w") as f:
+				f.writelines(lines)
+			print("The previous configuration inside the", backup_configuration_file, "file has been commented out.")
 
 ###########################################################