One thing worth mentioning:
If the ring carries a Hallmark it is, in fact, illegal to take it out when the work is done. The only reason I say that is because I had a ring enlarged and in the process the gold was 'smeered' over the Hallmark. It was a good few years old so carried a full Hallmark. I was compensated for that and the jeweler in question was given a 'slapped wrist.'
As far as value is concerened it may well not increase it's value, but only because the weight of added gold would be minimal. Providing the work is carried out well it won't even be noticed it's been enlarged, so there is no way it can lose any value.
~F~