Ok here is an effect (a mask) that will help you achieve that in seconds using just FCP.
I called it Bars Wipe but it is essentially a mask that you want to apply to a text generator
so that everything behind (i.e. in V1) remains visible.
Like here:
All you have to do is install the plugin, restart FCP apply it to the text, which I will assume
is going to be in V2 and adjust parameters to taste:
Keyframe the In/Outwards parameter to close the bars, cut the text generator
at the point the bars close together, change the text in the following clip and keyframe the
paramater again to open the bars. Done.
Example:
http://www.videonline.co.uk/Forums/Bars_Wipe.mov
Plugin do download:
http://www.videonline.co.uk/Forums/Luca%27s%20Bars%20Wipe%20%28Mask%29.fcfcc.zip
Installation instructions:
After downloading the plug-in drag it to Macintosh HD / Library / Application Support / Final Cut Pro System Support / Plugins folder. Finally restart Final Cut Pro and the effect should show up in the effects menu (Luca's Plugins).
hope this helps,
Luca