I don't think so.
People do go through changes and some of them have changed quite a bit.
Amy came in as a female Sheldon but her character expanded (thankfully) and they rewrote her. Maybe they realized that having the same character just with different genders wouldn't work.
If you remember Sheldon was also a littbe bit different in the pilot episode.. he even tried to hit on Penny but they changed his character too a little bit.
Howard evolved from creepy nerd to married and less creepy, just nerdy.
Raj got over his speech problem when in the vicinity of women (finally) and is now trying to find a partner.
Leonard has become a little bit more sure of himself especially when it comes to Penny but basically he's still the most down to earth guy that he ever was in that group (though still a full on nerd).
Sheldon has also developed over time (though at a glacial pace), accepting Amy into his life and starting to call her his girlfriend and most recently initiating the first kiss (which he apparently liked very much).
So most of them have progressed and changed with life and circumstances while still retaining most of their basic character traits. Sometimes they surprise us (such as Bernadette having this horrible competitive streak) but often enough they stay to their roots.