Hi,
You can do a "commit synchronize" on router having both REs on different version, but it is not recommended. The 1st option which you listed is much safer. You can save the configuration on Master RE in a file and then insert the new Backup RE and copy the config file from Master RE to Backup RE something like this:
file copy /var/tmp/currentconfig.tx re1:/var/tmp/
assuming re1 is backup RE.
Also important point to note is, when you are performing RE replacements, it is always recommended to deactivate graceful-switchover and nonstop-routing when both REs are on different Junos versions.
Hope this helps.
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