There is some tcl available to do this. Save and unzip the attached file to <your_install_dir>\tools\capture\tclscripts\capDB folder. Then launch Capture and open your design. Run a Command Window (View - Toolbar - Coomand Window) and then type:-
package require capDesignUtil
Then to replace offpage text in the currently active design::capDesignUtil::replaceOffPageText <pOffPageToSearch> <pOffPageToReplaceWith>e.g.:- ::capDesignUtil::replaceOffPageText _3 _4