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 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.

Edit

You're right. How about

sed -r 's/(John Smith)/temp1/g;s/Smith/John/g;s/temp1/John Smith/g'

This seems to do the trick.

 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.

Edit

@manatwork,gilles

You're right. How about

sed -r 's/(John Smith)/temp1/g;s/Smith/John/g;s/temp1/John Smith/g'

This seems to do the trick.

 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.

 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.

Edit

You're right. How about

sed -r 's/(John Smith)/temp1/g;s/Smith/John/g;s/temp1/John Smith/g'

This seems to do the trick.

I want to replace all occurrences of a character's last name with their first name, but only if their first name doesn't come immediately before their last name.

Why don't you find "last name + first name" occurrences and just delete "last name" ? Seems you are over-complicating it.

sed -r 's/(John) Smith/\1/g;s/(Jane) Johnson/\1/g'

 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.