I have a pair of custom ear molds for the Shure SCL4 earphone drivers. Can I re-use the custom molds on the new Shure SE425?
Sorry, in almost all cases, the answer is "No."
A custom mold is fitted to your ear canal and to a specific earphone driver. Just as your custom mold will not fit your band mate's ear canal, your custom mold will not fit a different earphone driver.
The same is true if a person has custom molded earphones with interchangeable attenuation discs for hearing protection. The "hole" in the mold for the attenuation disc will not properly fit an earphone driver.