Can a guilty plea in a Pennsylvania case 17 ½ years old of simple assault be reopened for any reason? Or is it final - over and done?

The United States Constitution prohibits "double jeopardy", i.e. being prosecuted more than once for the same crime.  Theoretically, you could be charged with some sort of ancillary crime, such as perjury or jury tampering, but after 17 years the statute of limitations on these crimes would have long passed, as would (absent unusual circumstances, such as the victim being a very young child who has only recently obtained adulthood) the statute of limitations on any civil claims.