If you're increasing the cost of your trip then there's no fee to be paid! I think further back in the thread someone suggested booking one less night than you need and then if you need to change dates due to flights etc you can add on that night to suit, which seems sensible.
Think the amendment fee is about £50, but don't quote me on that! I'd assume if the cost is the same you'd still have to pay the fee... but again I'm not certain, logically that makes sense to me though.
I shifted our October stay back by 1 day, cost stayed the same, and didn't pay an amendment fee (WDTC). I'd already linked it in MDE, which I was dreading a bit, but the change showed up there about 48 hours later.