SSMS 17.1 stopping autocomplete suggestion of variable types
For example, if i type NVARCHAR, the autocomplete suggests nvarchar
  and once i continue typing, it automatically correct the word.


This autocompletion is not just a "convention", SQL Server types that you can see querying sys.types are all lowercase, and this is a good reason to use them ALWAYS lowercase:

if one day your code migrates to a server with case sensitive collation, no one of your UPPERCASE types will be recognized as system type.

So yours is not a good convention and I think you cannot change this at least in SSMS because they(MS) are aware of this possible issue.

You can look for some customizable code formatter in internet