As per the provided documentation, it's a relatively trivial process.
Most of the configuration is pulled down to the new MX after replacement. Only things like port specific configuration and Site-to-site VPN config will have to be re-configured.
The above comments are excellent. The one thing I'll mention is any WAN traffic shaper you have configured on the MX68 (under Security & SD-WAN/SD-WAN & traffic shaping) will be kept - whereas you often want to use the higher performance option offered by the MX105.
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