'Still the Beaver': a return visit

America has gone through enormous changes in the past 20 years, but the town of Mayfield, TV-land, remains the same. Just older. As are Tony Dow and Jerry Mathers, the co-stars of ''Leave It to Beaver,'' the all-American, perfect-nuclear-family situation comedy that went off the air in 1963, after a seven-year run on ABC.

Now in their mid-thirties, Tony and Jerry - that is, Wally and Beaver - are back again in good old Mayfield: Still the Beaver (CBS, Saturday, 9-11 p.m.). Using the original actors, this special picks up the Beavers (all except Dad, who is supposed to have passed on in the interim) and projects them into the kind of life they might well be living today.

It's a joyous revel in nostalgia for those who remember the family way back then. But, it's also a reminder to all of us of how squeaky clean - and still entertaining - a TV series can be when it concentrates on an average family trying to fulfill the American dream. After watching the two-hour show, I felt as if I had just taken a refreshing shower.

Not that ''Still the Beaver'' is all peaches and cream - there are problems, such as Beaver's separation from his wife and attempt to keep custody of the children. Beaver, you see, remains a good-natured bumbler, always the kid. But this show gives him a chance to mature. And then there is Wally's tension-ridden desire to start a family - and the positive psychological effects of nurturing Beaver's kids.

The neighborhood friends are also included - now grown up, but still recognizable not only by their faces but by their amazingly consistent characters. You remember Eddie Haskell and Lumpy Rutherford, don't you? But most important, there is Mom, still played by Barbara Billingsley, now adorably graying.

An amazingly deft script by Brian Levant, directed with traditionally simple sitcom magic by Steven H. Stern, manages to bring all the characters up to date, allowing them to grow just enough so that Mayfield isn't frozen in the 1960s.

The script allows everybody to grow. Tony and Jerry have aged well - both a bit plumper, but both managing to re-create their characters with as much charm and honesty as in the old days. ''Beaver'' buffs will especially appreciate the fact that the special is dedicated to the memory of the actor who played ''Dad'' in the original series, the late Hugh Beaumont.

I won't tell you more of the plot since, as always in the case of ''Beaver'' skits, the story line is just a secondary device. What's important is the valid motivation and inter-reaction within the family.

Wouldn't it be nice to have the Beaver family back on television regularly, just as they are presented in this special? Certainly CBS and Universal TV are harboring the hope that viewers will demand more.

If American TV holds true to its follow-the-leader principles, the success of this special could trigger a rash of revivals of those healthy family series of the 1950s and '60s. So if ''Still the Beaver'' gets high Nielsen ratings, you can look for ''Still Ozzie and Harriet,'' ''Still Room for Daddy,'' ''Still the Many Loves of Dobie Gillis,'' ''Still My Three Sons,'' and (please forgive me) ''Still the Beverly Hillbillies.''