Just a thought- can you determine the two lengths you want, and make the depth of the folded part of the longer hem the same as the difference between the two hems? That way, with the slimmer fir, you might be able to just turn/roll under to shorten, and the weight of the fabric will give a nicer look and hold better.
I'm not sure I'm explaining well. It will work best if the difference/hem depth is not more than 2"' and the ankle is not too tapered. Also, I'd probably serge the inner edge and blind stitch the hem so it is 2 layers when down, and 3 layers thick when rolled under. If you double turn the longest hem, it might be too thick when rolled under.
I'm just think out loud as I write this, so I hope my stream of consciousness makes sense.