I have the same problem, and have been searching for a means of changing the Command Codes, with no success. In addition, original ADP-036 remotes are very hard to find, and expensive. There are lots of universal remotes, but none that allow for the command code change.
I did find one old Sony remote which performs most of the important functions of the ADP036, BUT only if the Sony 400 player is set to Command Code 1
The remote is the Sony RMT-B103A (I don't know which Sony Blu-ray dvd player it was meant to control--I've owned many over the years).
Its quite inexpensive and readily available.
Otherwise, the only solutions I can think of are pretty obvious (1) find out how to add the command control function to the B103A, or (2) Reset a fully-functional ADP036 remote to Command 2 or 3, and then use the B103A, which is always set to Command Code 1, on the player which was originally on Command Code 1 [this allows you to keep one fully-functioning ADP036 for in reserve for future use] 0r (3) buy an original, fully-functioning ADP036.
A downside with this interchange of course is that the B103A may unintentionally turn on other unrelated Sony devices. But you can obviously put a removable cover over the small sensor on the front of the unintended device, and remove that cover to use the device. Or rearrange competing devices so the two are far enough apart to not easily simultaneously pick up the signal from the remote.
There may be other Sony remotes which function as substitutes as well as the B103A, and I hope people will post any additional information or insights.