Can I use a 2.0 script in a 3.0 script Flash file?

Question

The following script is a simple photo gallery, written in Action Script 2.0. When I attempt to apply the same script in a Flash file designated as having Action Script 3.0, it does not work properly (the photos advance from frame to frame in a runaway fashion, in one continuous loop).
Is there some reason why a 2.0 script should not work in a 3.0-specified document?
Thanks, and here is the script:
stop();
btn_next.onRelease=function() {
    if (_root._currentframe==20) {
        gotoAndStop(1);
    else {
        nextFrame();
btn_prev.onRelease=function() {
    if (_root._currentframe==1) {
        gotoAndStop(20);
    else {
        prevFrame();

Answer 1

there's no easy way to convert most code.  but that's almost the same in both as2 and as3:
stop();
btn_next.addEventListener(MouseEvent.CLICK,nextF);
function nextF() {
    if (MovieClip(root).currentFrame==20) {
        gotoAndStop(1);
    else {
        nextFrame();
btn_prev.addEventListener(MouseEvent.CLICK,prevF);
function prevF() {
    if (MovieClip(root).currentFrame==1) {
        gotoAndStop(20);
    else {
        prevFrame();