Replace line in file based on pattern

[Paul M. Foster]

Folks:

In a script, I'd like to search for a pattern in a file, and replace 
that line entirely with a new line, once (not globally). I've tried

sed -i s/search/new_line/

but this only replaces the string itself. I want the script to find the 
line my search term is on, and replace the whole line with my 
replacement line.

Anyone know how to do this?

Paul