to make it smaller the goldsmith removes a little section of the ring, preferably on the side thats facing the palm. to enlargen he either adds a piece (also usually facing the palm) or if the ring is wide (like ‘chunky’ish) enough uses the existing material making the ring itself slightly thinner.
engravings would have to be redone but thats no big deal really.
Its not really possible to keep the spacing right in the stated case. depending you have to either get a new ring and reuse the diamonds, have a goldsmight add a layer of gold or whatever metal to the inside of the ring (in an emergency oneself could add a layer of hot glue instead to make it fit better, tho maybe not on a diamont ring) or wear it behind a ring that actually fits. you can show your ring to your nearest godlsmith and he can tell you what is best
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