Have you been using JIRA for more than a year, or are you a power user of JIRA? That is, have you performed huge customizations, created numerous plugins, used lot of use cases, and so on? Then it is very likely that you have come across this scenario. You want to move the values from an old custom field to a new field.
JIRA doesn't have a standard way of doing this. But you can achieve this to an extent by modifying the JIRA database. Even with SQL, there are some restrictions on doing this.
The first and foremost thing to check is that both the fields are compatible. You can't move the values from a text field to a number field without extra checks and validations. If there is a value 1234a stored in one of the issues, it can't be stored as a number field as it is not a valid number. The same applies to all the field types.
Let's look at the migration of compatible types and discuss a few other scenarios in this recipe.