There is no automated way to do this via the PCB Interface but you can (probably) do this manually using the Add Properties command. This command works in two modes 'Object-Action' and 'Action-Object'. To achieve what you want you can use either method, in 'Object-Action' mode select the components you want to modify (push into the block in the schematic so you're in instance mode) and then open the dialog (toolbar button with a component and A+). Select Type= 'Component', Property=REFDES, 'Instance Values'. Choose 'Range', Prefix = CH1_n, Value 1, Delta 1 and Apply. This will apply REFDES of CH1_n1, CH1_n2 etc at the instance level starting from top to bottom or left right in the selected objects. The PCB interface should not override these settings (it hasn't in my simple test which is why I said probably above).
If you use 'Action-Object' mode you first open Add Properties and fill in the dialog in the same way but instead of clicking Apply, the property is on the mouse cursor, then you select each object in turn to apply the REFDES property, this allows you to assign REFDES out of sequence, unlike 'Object-Action' mode.
I hope this helps.