Welcome to FT!

If you have been some level of Medallion since the old program (1995) with no breaks (years without elite status), you can use old miles for awards from the old Medallion award chart. Since under its old program Delta gave elites awards for fewer miles instead of giving them bonus miles but using a single award chart, the old chart is more generous. (My favorite was the 2500-mile round trip upgrade from any fare.)

Other than that, and not expiring as you mentioned, there's no difference.

In the future, you'll probably do better on airline-specific questions such as this in the airline's own forum.