DCB0

We have different opinions (I am LarDasse74). Mine more from experience working in a shop and designing & helping customers deign hundreds of custom built and assembled bikes over the years; cycos developed by years and years of reading internet forums (jk)

In reality, his opinion is as valid as mine, and, as with anything you read on the internet, it is up to you to separate the wheat from the chaf. While I hesitate to move the saddle to adjust reach, I will not say that it never works or is always a bad idea. If it makes the bike feel better, then go for it. Then changes you are talking about (25mm setback to zero setback) is not a major change. Also, I just checked the geometry of the Cross CHeck on the surly website and the 54cm has a slightly steeper seat angle than the 56cm... if you have your saddle height at, say, 64cm from the bottom brakcet, the 1/2 degree of difference between the two frame sizes will result in 0.5cm (5mm) difference in seat setback, while swapping the seatpost will result in a change of 25mm of setback.

My hesitation in reccomending saddle adjustment to fix a reach problem is that moving the saddle changes how and which leg muscles are used, not just the reach.

But really, it's not that big of a change. And the top tube length of the 54 is only 10mm less than the 56, so if you had started with the same stem length that you originally had on the 56, you would have started with the same stem reach you have now less 5mm. Like Cycocommute said, a new post can be dirt cheap and is certainly worth a try.

But speaking as someone who (on more than occaision) has tried to get a bike that was only one size off to fit me (I am quite tall and most manufacturers' largest bikes are borderline unacceptable for me) and failed, or got it as close as I could and it still didn't feel right, I think you should start planning to replace the frame with the correct size.