I am still not clear why your update is not working. One thing, your case statement should be changed
update dbo.BigTable set BigTable.Software_Version_Raw =
case
when CHARINDEX('.',Software_Version_Raw,0) >0 then
substring(Software_Version_Raw,0, CHARINDEX('.',Software_Version_Raw,0) )+ '.x'
when isnumeric(Software_Version_Raw)=1 then
Software_Version_Raw + '.x'
else
Software_Version_Raw end