You can keep your existing config.json file in v1.2. However I would recommend that move your config to environment variables as @ianbmacdonald said. Note that certain options are not available as environment vairables but have instead been moved to the databsae (under admin -> config tab). Here’s the full list of these options and their equivalent in the database:
DOWNLOAD_TIMEOUT: cwmp.downloadTimeout
DEBUG: cwmp.debug
RETRY_DELAY: cwmp.retryDelay
SESSION_TIMEOUT: cwmp.sessionTimeout
CONNECTION_REQUEST_TIMEOUT: cwmp.connectionRequestTimeout
GPN_NEXT_LEVEL: cwmp.gpnNextLevel
GPV_BATCH_SIZE: cwmp.gpvBatchSize
COOKIES_PATH: cwmp.cookiesPath
DATETIME_MILLISECONDS: cwmp.datetimeMilliseconds
BOOLEAN_LITERAL: cwmp.booleanLiteral
CONNECTION_REQUEST_ALLOW_BASIC_AUTH: cwmp.connectionRequestAllowBasicAuth
MAX_COMMIT_ITERATIONS: cwmp.maxCommitIterations
DEVICE_ONLINE_THRESHOLD: cwmp.deviceOnlineThreshold
UDP_CONNECTION_REQUEST_POR: cwmp.udpConnectionRequestPort
The value should be an “expression” which is a new format that I haven’t yet had the time to document. For now you’ll probably get by using numbers, boolean (true/false), or quoted string literals ().