Originally Posted by darkmother

Should be fine. Most cross forks are around 400-410mm axle to crown. Modern rigid MTB forks are typically longer than that, to match 80 mm travel suspension forks. Really old school rigid MTB forks are around, or slightly under 400 mm IIRC. You are probably talking about steepening your head and ST angles by less than 1 degree, and maybe dropping the BB height by less than 1 cm. Both things are no big deal and should actually improve the handling a bit.