If you already have a Teradata platform, I think one good first step might be getting SAS/Access for Teradata and figuring out the appropriate libname statement to access your specific database (also contact the Teradata DBA to make sure you have permissions to read). I think you'll need that down the road anyway for what you're proposing.
http://support.sas.com/documentation/cdl/en/acreldb/61890/HTML/default/a001384369.htm
The next step would be to see how much modification some of your existing flat file to SAS data code would require now that you're connecting directly to the Teradata tables. If you have a lot of data manipulation (creating columns, cleaning data, etc.) in thousands of lines of SAS code you describe, you may still need some of that in the revised solution. If the code is mostly just to read the flat files, that part is obsolete with the advent of the LIBNAME statement for Teradata. The reality of all ETL tools is that cleaning and prepping the data will take time, system resources, and usually require scheduling.
Data manipulation you can do in PROC SQL may be vastly prefered to Data step code since some/all of the PROC SQL code can be passed through to the database for processing.
Just a few thoughts from someone who is definitely not a data warehouse architect. So take this with a grain of salt...