Changing the guts replaces the HDD which includes latest 450r6.
If they have a 4.0, apply the latest system update to the 4.0 (or at least meet DMM minimum requirements), upgrade the licenses, extract the data off the 4.0 using latest DMM (NOT 1.1.0), prepare the data, upgrade the hardware in a spare chassis, apply the latest system update to 450r6 (or at least meet DMM minimum requirements), apply the "upgraded license" to 450, use DMM to reapply the data. Don't extract/apply the data through the OAM, use LAN port. Do ensure Unified Messenger keycode is applied to 4.0 (required for exporting the mailbox messages) before upgrading the license from 4.0 to 450 (so the 450 carries over the UM license).
If they don't want to upgrade, and just replace for a 450; guess it's your judegement/pricing on how long it will take to reconfigure.
If it's a 3.x to 4.0 to 450 upgrade, probably best to reconfigure a replacement 450, however its a real time consuming affair (2 days minimum) to extract off a 3.x because every button, speed dial entry, target line appearance etc etc is a click per parameter. I just hate those 3.x's for reconfiguring.