Use apenas um delimitador diferente:
sed -i 's|foo|bar|g' filename
Então, no seu caso:
sed -i 's|/home/saeid/public_html|/home/saeid/www/domain.com/html|g' file
Isso é mencionado em info sed
:
The syntax of the s (as in substitute) command is ‘s/regexp/replacement/flags’. The / characters may be uniformly replaced by any other single character within any given s command. The / character (or whatever other character is used in its stead) can appear in the regexp or replacement only if it is preceded by a \ character.