How can I seamlessly switch between IK/FK? How can I switch between IK/FK without having bones changing their transformation?
 I have forearm bone with IK constrained. Now I want to disable IK and rotate forearm.

 The problem is whenever I disable the IK constraint, the forearm moves to the side.
 How can I disable IK without losing position ?
Under the Rig Main Properties you have the IK/FK slider. Below that you have two options to snap the pose to either the IK or FK position.



p.s. I notice you are using the MetaRig to animate. Generally the MetaRig is used to generate a new rig with all the bells and whistles like IK/FK organised for you.



The 'generation' also creates the Rig Layers which makes switching and controlling easier...