I would use SELECT to make sure that the Replace was what I wanted and then use UPDATE once I had tested it. Something like this:
-- UPDATE C
, '"''John''"', '"John"')
, '''John''', '"John"')
FROM client AS C
Once the test is completed Comment-Out the SELECT line and Comment-In the UPDATE and SET lines.
You could also take a copy of the table, before you start, in order that you can "copy it back" if the update fails
DROP TABLE TEMP_client_20170614
once you are sure everything has worked OK. (I use TEMP prefix, and a date Suffix for this type of temporary table so when I see the table is still there in a couple of months!!! I can say "That's TEMP and OLD ENOUGH that I can't possibly need it any more, so I can DROP it now ...