First of all: The quoting is not correct (back qoute to open, single quote to close). Should both be single quotes.
Here's one solution using sed and taking the whole string (you migth be able to shorten the string, but do watch out for false hits)
# for sed versions < 4.0, but works with all sed version including 4+.
mv /etc/mail/sendmail.cf /etc/mail/sendmail.cf.org
sed "s/define('SMART_HOST','smtp.myisp.com')/define('SMART_HOST','smtp.ourisp.com')/" /etc/mail/sendmail.cf.org > /etc/mail/sendmail.cf
# The command below only works with sed => 4.0 (sed -V to check this)
# sed -i.org "s/define('SMART_HOST','smtp.myisp.com')/define('SMART_HOST','smtp.ourisp.com')/" /etc/mail/sendmail.cf
Make sure you have a backup of the original sendmail.cf before you start testing/trying.
Hope this helps.