With Shape Tweening you can change one object into another.

Step 1
Choose the Text tool in the left toolbox. Choose Text > Size > 48 from the top menu to make the text big. Choose Text > Style > Bold to make the text thick.

Step 2
Click in the work area and write "Hello".

Step 3
Right click on the text you just wrote and choose Panels > Align from the pop-up menu.

Step 4
In the Align box select the "To Stage" button first. Then click on the "Align Horizontal Center" button and the "Align Vertical Center" button. Close the Align box.

Step 5
Select the Arrow Tool and click on the text. Choose Modify > Break Apart from the top menu.

Step 6
Insert keyframes at Frame 24, 50 and 51.

Step 7
Delete the text "Hello" in Frame 24. Select it and press the Delete button on your keyboard.

Step 8
Write a new text on the Stage. Write "World!" (Font size: 48, style: bold).

Step 9
Right click on the text you just wrote and choose Panels > Align from the pop-up menu. In the Align box select the "To Stage" button first. Then click on the "Align Horizontal Center" button and the "Align Vertical Center" button. Close the Align box

Step 10
Select the Arrow Tool and click on the text. Choose Modify > Break Apart from the top menu.

Step 11
Insert a keyframe in Frame 26.

Step 12
Double click the keyframe in Frame 1. In the small pop-up box click on the Frame tab. Set Tweening to Shape. Close the pop-up box.

Step 13
Double click the keyframe in Frame 26. In the small pop-up box click on the Frame tab. Set Tweening to Shape. Close the pop-up box.

Step 14
Double click the keyframe in Frame 51. In the large pop-up box click on the Frame Actions tab. Click on the + sign. Choose Basic Actions > Go To. Close the pop-up boxes.

Step 15
Choose Control > Test Movie from the top menu to test your Flash movie.