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Is that 18mm difference going to be a problem? I assume the answer is yes!
It will make a difference. Whether that amounts to a problem is another matter. Do you want to steepen the head angle by 1°, reduce the trail by 7mm and lower the BB by 8mm? That still leaves well inside the range of ridable bikes, assuming your starting point isn't anything radical, but you can avoid doing any of that by picking a 400mm AC fork, which is a common enough size, e.g. Surly Straggler
I would like replace the fork on my partner's bike - current fork has a 45mm offset and 401mm axle to crown.
Eyeing up the Thorn Disc Steel Fork which has offset of 46mm (assume that's fine) but is only 383mm axle to crown.
Is that 18mm difference going to be a problem? I assume the answer is yes!