A large file, and no powerful enough editor at hand to perform a search/replace?

It's simple with vi.

Edit the file with the vi command
vi file

Then in the editor type:
:%s/mystring1/mystring2/

Remplacer une chaine dans un gros fichier avec VI
Remplacer une chaine dans un gros fichier avec VI

Alternatively, there is another option with sed:
sed -i -e "s/mystring1/mystring2/g" file

If the string contains special characters, they will need to be escaped with a backslash '\'

sed -i -e "s/\#mystring1/\#mystring2/g" file