It will be close but the cooldown period will end before the 18th Jan. You said 42 days left. 3 days remaining this month. December has 31 days. The the 18 days in January. So more than enough. Ps. The information about Stadia and the advice from Bungie not to disable cross save was released less than 2 days after the Stadia announcement.
Please go to this article and fill out the form: https://help.bungie.net/hc/en-us/articles/10537372531092 . It is specifically for those people that have Stadia as their primary save data and are locked out of making changes. While you may have just enough time to skirt by the wait period before the deadline, I would suggest using the form in order to ensure that you don't miss the deadline and forever lose your save data.