Really? Where did you get this information?
The upgrade was announced on board, and I am sure I was either told - or read somewhere - that the price is the same as Weekend 1st, which has just been reduced to £15 from London to York. I can't find any mention of this on the East Coast site.
Edit: After extensive research, I found that
YorkshireRider (who works for EC) reported back in May that he heard it was the same price as weekend 1st (which
was £25 to York/£20 to stations to Doncaster but is reducing to £15). If it was the same as Weekend First that would reduce it to £15 in the new fares,
however he posted a correction stating that it's a "flat fare" of £20. Therefore, it may not have had a reduction, so may remain at £20 rather than £15. (in my defence I did say "I think" as I wasn't sure of the exact amount
) If it does remain at £20, that means it would previously have been cheaper to upgrade during the week than at a weekend to York (yes, really!), but would now be cheaper at a weekend.
So, all I can say for certain is that the upgrade to York was £25, and £20 during the week, but is now going to be £15 at weekends, and perhaps still £20 during the week or maybe that's reducing to £15 as well, I do not know.
Either way, it makes a separate Standard Advance + upgrade cheaper - though with absolutely no guarantee of a seat in 1st, though the York terminators will be quiet in 1st so you should be fine on those during the week. I do not know why you are so shocked that splitting may be cheaper though