The "TECH 1A" was the GM dealer scan tester from 1986 up to about 1996 or 97.
It cannot program calibartions (chips) used in ECMs up to 1993. I believe a T1A, with the proper accessories, could flash the EEPROMs in 94-97 ECMs with a new calibration the user supplied.
I think it can also program CCM after a CCM change.
Note the words "could" and "I think" above. i say that because it's been more than a decade since I used a T1A in the shop and I sold mine back about 2002, so I can't remember exactly some of the procedures. I'll add that the T1A is no longer supported nor are accessories or parts still available so anything you find is going to be Craig's List/eBay type items.
As for the question about a device which can both "scan and modify"...if you mean a hardware device, the answer is "conditional yes". There are a few high-end scan testers, such as Bosch TECH2 (see this web site
Tech 2) which is the current GM dealer tester, which can both act as scan tester and can reprogram 94-up ECMs, however, they cannot write the software code for the calibration. You have to have that from a dealer or have to write it with another app, such as "HP Tuners", then bring that document into the T2 which flashes the EEPROM in the ECM.
There are also some software based apps which run on a PC which both help you write the cal, again, I suggest "HP Tuners", and with the addition of some hardware accessories, allow you to get the program from your PC into the ECM.
There are, also, PC apps that act strictly as scan testers and EASE is the best of them and the one I use when I use a software based tester. For more info, go to
EASE DIAGNOSTICS