Jenny,
I personally do not advocate the replacement of stretchers. Especially in old paintings. Let the curve will be, but all the same "family". In your case the picture otonositelno "fresh", 1971, and if the author is not an artist of the first magnitude, but can change. By the way good bagetchik can perfectly fit the frame of a curve stretcher and nothing will be noticeable if the course is not much to look closely
And if a stretcher is copyright tagging tags auctions, etc., then it is better not to change.