Hi [...],
The best way IMO to substitue would be to change .125mg K with 2.5mg valium once a week for 4 weeks. Like this:
Week 1 = daily dose would be .375mg K/2.5mg V
Week 2 = daily dose = .25 K/5mg V
Week 3 = daily dose = .125K/ 7.5mg V
Week 4 = daily dose = 10mg V
After stabilising on the 10 mg for 1-2 weeks you could then start to taper off the V by 10% every 7-10 days depending on how you feel.
You will have some cutting of pills to do though, so if you don't have a pill cutter then you would need to get yourself one.
The .5 K can easily be cut into 1/4's to give you the .125mg and the same with the 2 mg tabs of V to give you .5 mg V.
As you only have a finite supply you would need to save up all the little 1/4's of pills as you will need them further down the track.
Try and be as economical with them as you can.
Good luck