There are two banks in the game that will change your old money into spendable Crowns if you ask them. There’s the Bank of Vivaldi on Heirarch Square in Novigrad, and there’s the Cianfanelli Bank at The Gran’place in Toussaint.
I was wondering if one paid out more than the other, so I ran a test to check, and it turns out that they both give the same amount of gold for conversions, so you can use either, safe in the knowledge that you’re not losing out.
The Witcher 3 Guide 