Using the .step command to step a parameter of a transistor model in LTspice
Jramsay42 is correct.

See the help to .STEP:


  .STEP -- Parameter Sweeps This command causes an analysis to be
  repeatedly performed while stepping the temperature, a model
  parameter, a global parameter, or an independent source. Steps may be
  linear, logarithmic, or specified as a list of values.
  
  Example: .step oct v1 1 20 5 
  
  Step independent voltage source V1 from 1 to 20 logarithmically with 5
  points per octave.
  
  Example: .step I1 10u 100u 10u
  
  Step independent current source I1 from 10u to 100u in step increments
  of 10u.
  
  Example: .step param RLOAD LIST 5 10 15
  
  Perform the simulation three times with global parameter Rload being
  5, 10 and 15.
  
  Example: .step NPN 2N2222(VAF) 50 100 25
  
  Step NPN model parameter VAF from 50 to 100 in steps of 25.
  
  Example: .step temp -55 125 10
  
  Step the temperature from -55°C to 125°C in 10-degree step. Step
  sweeps may be nested up to three levels deep.


Which works as you expect it. You also don't have to replace the value of your model by {BF} in that case, it will just overwrite the value of the model.