I am looking into the capabilities of the Box Governance add-on.
I have have been reading the tutorials and see that there is a rule that says 'If a user moves a file carrying a Retention policy to a folder with a different retention period, the longer retention period will supersede the shorter one. The retention period will be based on the upload date of the file, not the move date to the new folder.'
However, in the tutorials for creating retention policies the instructions to retain records for as long as a client is current, then for 6 years after they leave, involve applying an indefinite time period to a 'Current' folder and a 6 year time period to an 'Archive' folder.
In this case, according to the rule above, if a file is moved from the 'Current' folder to the 'Archive' folder it will retain the indefinite retention time period.
The only way I can see to deal with this would be to copy the file from the 'Current' into 'Archive' then delete the original.
I managed to find some information to answer my own question. There are some exceptions:
Moving a file from a folder with an ‘indefinite’ retention period to a folder with a ‘finite’ retention period. The file is then subject to the finite period starting from the date of the move.
Moving a file from a folder with an ‘indefinite’ retention period that had previously been located in a folder with a 'finite' retention period longer than the new destination folder. The file is subject to the original (longer) retention period, based on the date it was uploaded or added to the original folder.