I wouldn’t say it has to be done manually, no. When using the ‘More Options’ button in the Edit Computer settings dialog, you can designate a Target set of Clients the same way you would with an action taken by Fixlet (i.e. select devices, dynamically by property, or paste in a list of device names). So, one targeting method you could use is to select (or dynamically target) Clients by ‘Relay’, and select the Clients reporting to the ‘old’ Relay to point them to the new one. Below are some example screenshots:
// Edit Computer Settings -> More Options, and set the desired Relay settings

// For the Target, select the Clients reporting to the old/previous Relay (either specifically, or dynamically)
