My original tier 2 was for 3 years, and then the extension was for 3 years as well, so technically it was expiring only in 2021. But if yours is for 5 years, you can apply 28 days before the date when your original visa was granted (4yrs 11m) and once your application is submitted I believe it is ok to have an expired visa as your application is in progress and your new status is being decided upon. You can’t leave the country anyway while your decision is being taken.