How hard would it be to come back programming in Cobol after over 25 years without touching it?

Answer 1

You could still develop applications using COBOL 68, but you'd lose a lot of functionality if you didn't familiarize yourself with many of the newer keywords. For example:

EVALUATE allows caselike structures, avoiding nesting IFs testing the same field for various values

Many keywords now have corresponding END statements such as END-IF, END-READ, END-PERFORM, etc. Among other things, PERFORM/END-PERFORM allows in-line looping instead of invoking a separate paragraph, thus possibly avoid page swaps during execution.

COBOL 2002 added OOP aspects to the language.

There are now varieties of COBOL designed for desktop applications, including COBOL.NET which gives access to the whole .NET foundation libraries.