Lightroom - Simulating different focal lengths

If I have a photo taken at 250mm, how can I crop that photo so that it would give me what I would have captured using 400mm (or any other focal length)?

I am looking into getting a new telephoto lens (100-400mm or 150-600mm) and I am trying to see with the photos I normally take how different the image would be at a bigger focal lengths.

I hope this makes sense to someone.

THIS tool might be of assistance in enabling you to see the differences at varying focal lengths.

250/400 = 0.625

For 3000 x 4500 pixels image:
3000*0.625=1875
4500*0.625=2812
Crop to 1875 x 2812 pixels.