It depends on the position of the post-function.... if it's near the top of the list, you can retrieve the issue, before modification, from the database:
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.IssueManager
def issueManager = ComponentAccessor.getComponent(IssueManager)
def originalIssue = issueManager.getIssueObject(issue.id)
If you need it after the functions which save the issue to the database, either retain a reference to it from an earlier post-function, or use the ChangeHistoryManager to see what changed.
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