I have done it on a TDS544a which is very similar to your TDS540A. Enabling the options is really very easy if you have GPIB connection. Unlock the scope with the password string then enable the options one by one.
Tektronix forums has all of the details here
Unfortunately, I don't have a GPIB adapter.
I looked at the topic you linked, and now I am confused. They are talking about 1245Y chip, and also about 1486, which they propose to read as 1245. Is it the same chip?
In my two 540A scopes there are two chips: 1650Y and 1286.
Which one has the options?
I backed up 1650Y as 1250Y - is this the right way?
How can I read the 1286 (TL866 programmer doesn't support 1286)? Do I need to?
I also discovered that the backup file I made a few years ago of 1650Y from my older 540A looks quite different from the backup I made from the scope in the repair video. I used some other programmer, which I borrowed from a friend.
So I opened up my older scope and read the 1650Y again using the TL866 programmer. The result is very different from the old backup, and looks similar to the dump from the scope in the video.
Now I wonder if any of the dumps are correct at all.