The best thing to do is turn off Machine B or disable the network access and restart Machine A this should clear the Licence Key Conflict.
It is always best to uninstall when moving FileMaker Server to another machine on the the same network to stop getting the Licence Key Conflict Message
Install the trial version on the new machine. When done, de-commission the old machine and transfer the production (or in your case, development) install code to the new machine.
I first started computer_b while computer_a was off and uninstalled FMS on computer_b. Then when I started computer_a, all was fine.