Technically yes, it can be done but it is all a question of scale. Large gold refining companies do this on a daily basis. They are able to melt the gold at extremely high temperatures (around 1064°C) & then separate the pure gold from the other metals used to strengthen it using nitric acid. This gives them pure gold that can then have the appropriate alloys added to make it into white gold. These alloys are typically a combination of palladium & silver or nickel, copper & zinc.
Doing this on a smaller scale though is unfortunately not economically viable. The time it would take for a goldsmith to carry out the above process would mean that the labour costs would be excessive. There is an answer to changing your old yellow gold into white gold though…
To avoid our customers paying excessive labour costs at RING jewellers we tend to give the following options: If a customers has an old yellow gold piece of jewellery that they would like to remodel into a new yellow gold piece we will physically melt it & reuse it. If a customer has yellow gold piece & they want to end up with white gold we will make a financial allowance against the old yellow gold. Obviously this does not help with items of sentimental value but it does help when it comes to budget.
Another question we are frequently asked is if we can melt & reuse platinum & white gold. Although these metals can be melted relatively easily they are prone to splitting & cracking when reworked. For this reason we tend to make a financial allowance against these metals rather than physically using them. If the metal is being reused for sentimental reasons we can sometimes make a small feature on the item of jewellery. This means that there is a specific part of the design that the customer knows was made of their family heirloom.