Apply PR before starting of express entry

[jes_ON]

yes it would.. I would suggest send them updated letter after few months,, stating any changes if made to your salary...

I would not suggest this. You cannot "Update" the information in the sense that the VO cannot consider any new information or experience that occurs after the application is submitted.

The Letter of Employment must be dated AFTER completing the experience requirement. It is your key piece of evidence, your employer's testimony that you have the required experience. VOs don't entertain predictions about the future ("he will work until...)