after upgrading to 5.4.2 (form 126.96.36.199) when editing old imposition (Edit an Imposition Preset) in Press Sheet Size there is "Statement (5,5 x 8,5in)" and there should be custom sheet size (in this case C1max (330x488mm)). I have 100 imposition nodes to edit...
Custom press sheet sizes should be preserved across migrations. Can you contact Xerox customer support so a problem can be generated? Thanks.
We tested the scenario that you described. The custom press sheet sizes were retained across the migration. We did notice that the Custom Page Preview sizes (see screen snap showing 188.8.131.52 setup) were not retained across the migration -- and have written a BUG against this.