Hello Steve,
May I suggest a procedure to achieve this?
First of all, you will have downtime during the switchover.
I would use a script to:
1: remove the ip adresses from the FE
2: create the port channel
3: assign the same IP's to the port channel
4: make the existing FE part of the channel
Paste this script to the console. After this it should be possible to add a second link to the channel. With careful preparation (i.e. a script with no errors) you can make the switch witin a minute. In my opinion, this is the safest method.
Regards,
Leo