Over the years the Washington theatergoer has become expert in tryout-watching, that period when new ventures are prepping for all-important New York. Changes either will strengthen weaknesses or destroy promises.

Into the latter, unfortunately, falls the National's "Hellzapoppin." During its four weeks, the Jerry Lewis revue, a loose retread of the old Olsen-Johnson hit, presumably has been laboring to build on a 50-50 chance. It's not working and "Hellzapoppin'" is now a less enjoyable escape-from-life than it was opening night.

The changes are minimal, the pacing is a drag and co-star Lynn Redgrave, despite vows she'd have more to do, is absurdly wasted. Though fans' affections gave Lewis the standup treatment at the mid-week matinee, this was a salute to what he represents, not to his performance.

A new sketch gives him a quick-change change but it's poorly staged. Ownen McGivney, vaudeville's tops in this field, finally changed his act to let us watch him change. Why doesn't Lewis? Something's happened to that "Once I've Got My Cane," the Act I finale. Donald Saddler's choreography originally seemed to have more bounce.

Redgrave is a comedienne with proven potential, as she shows in her Bobwas Watahs bit, and what's more she's beautiful. Were she playing the usherette who barges into a scene of her own, she'd give glamor and sparkle to what continues to be chintzy and time-consuming. How ridiculous to find a performer with such style relegated to "second woman."

With such experts as Herb Edelman and Brandon Maggart, who are very good, and such clowns as Joey Faye and Justine Johnston, who are pretty much wasted this should have improved not retrogressed. Unless miracles happen during the coming Boston run. Alexander H. Cohen's expensive venture will have wasted its vital tryout period.