risk redo root canal?

[michaels01]

my dentist says my root canal done years ago is not deep enough, should be like 1-2mm before the end of the tooth. The tooth is a front tooth, already has a metal post and crown on it. what's the risk of redo the root canal? how risky is it to redo root canal with a metal post on it??

Also how do I knnow for sure I need a root canal redone? Sometimes I feel like there is a slight inflamation when i'm sick or when my immune system is weak, but most days I don't feel anything..