Hi [...],
Welcome to BenzoBuddies.
Switching from one benzo to another is not so mysterious. If you do not already have a prescription for the 0.25mg Xanax pills, I highly recommend that you do so. You should be able to split these pills in half. This means that you can split your daily dose of 0.5mg Xanax into four parts. You will also need 2mg tablets of Valium. To substitute, simply replace one-half of a 0.25mg pill (0.125mg) for an equivalent dose of Valium - Ashton would equate 0.125mg Xanax to 2.5mg Valium. Stick at this dose (0.375mg Xanax and 2.5mg Valium) for one week (or longer if you think you need the time). Then switch another half-pill for another 2.5mg Valium, so that your daily dose is 0.25mg Xanax and 5mg Valium. Again wait a week or more. Keep doing this until you have completely switched to Valium. At the end of the process, you should be taking 10mg Valium.
A variation on this would be be to obtain some 5mg Valium tablets in addition to the 2mg tablets. If you can quarter your 0.25mg Xanax pills, you could swap a quarter of a 0.25mg Xanax pill (0.0625mg) for 1.25mg Valium (a quarter of a 5mg pill). Again, wait a week, and then switch another quarter pill of Xanax for a quarter pill of 5mg Valium (1.25mg). Again, once your switchover is complete, you would be on 10mg Valium. You will still need the 2mg Valium tablets, as the 5mg tablets are too large when tapering from 10mg Valium.