Use global replacement in its interactive mode to avoid making changes that you don't intend. To do this add thec option to the end of the global replacement command:
:g/pattern1/s//pattern2/gc
vi will display the first occurrence of pattern1.
To replace it with pattern2 you must enter y (yes) and press <RETURN>.
If you don't want to replace this pattern press <RETURN> to go to the next occurence.