When another field is NOT NULL, is it possible to run a DATEDIFF else "some text"? 

Something like this:

CASE WHEN ("wos"."LAST_TECH_UPDATE" IS NULL ) then 'None' else DATEDIFF(day, dateadd(s,datediff(s,GETUTCDATE() ,getdate()) +(wos.LAST_TECH_UPDATE/1000),'1970-01-01 00:00:00'),GETDATE()) END "Request Overdue",

Thank you,

Jim