Hi SJ,
As suggested above you can still promote the Jobs from DEV to PROD; however, need to ensure that the amendments done in a newer version e.g. new features will not work in PROD.
In my opinion, 6 months is a long gap for upgrading the 2 environments. It will be more difficult if your client plans for another version upgrade within this period OR as suggested by Denis, call for a freeze period.
Another thing you can try i.e. promote one sample job from DEV to PROD and perform complete unit testing to check the working/ issues of this scenario.
I hope this will help.
NK