Simple, this can be achieved via “FInterpTo”. Just create a Branch with the conditional you are trying to use to change the camera boom length (ie. collision with box), if True set the Target Arm Length (using the Camera Boom as the Target) to the FInterpTo (Current = Target Arm Length), insert the length of your desired camera boom in Target and change Interp Speed to something like 0.45. Use GetWorldDeltaSeconds for Delta Secs input. If your conditional is False, do the same as above but under Target for FInterpTo, insert your original camera boom length. This will basically reset your camera boom length to the original when your conditional is false.
The example image attached was used for the velocity of my character. When the character’s speed was higher than 300, the camera boom length would get longer according to the velocity.