UpdateFields.groovy

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.ModifiedValue
import com.atlassian.jira.issue.util.DefaultIssueChangeHolder
import com.onresolve.jira.groovy.jql.JqlPagedResultsUtil
import com.onresolve.scriptrunner.runner.ScriptRunnerImpl

def jqlPagedResultsUtil = ScriptRunnerImpl.scriptRunner.getBean(JqlPagedResultsUtil)
def authenticationContext = ComponentAccessor.jiraAuthenticationContext
def customFieldManager = ComponentAccessor.customFieldManager

def query = "project = TCS and updated > -14d" // add other clauses if necessary

def originalId = 13206
def newId = 14646
def preview = true

def originalCf = customFieldManager.getCustomFieldObject(originalId)
def newCf = customFieldManager.getCustomFieldObject(newId)

jqlPagedResultsUtil.query(query).setUser(authenticationContext.loggedInUser).forEachIssue { issue ->
    def originalValue = issue.getCustomFieldValue(originalCf)
    def newValue = issue.getCustomFieldValue(newCf)

    if (newValue && !originalValue) {
        def changeHolder = new DefaultIssueChangeHolder()
        log.warn("Will modify issue ${issue.key}, last updated ${issue.updated}")
        if (!preview) {
            originalCf.updateValue(null, issue, new ModifiedValue(null, newValue), changeHolder)
        }
    }
}