Hi guys

I wanna change my suspension fork for a corrected fork.
So my actual fork has 475mm axle to crown and the corrected fork i am looking to has 440mm.

So i am asking, does the 35mm difference is normal when going from suspension to rigid fork?
Will it change something in my posture, may i get sore back?
Should i get a different stem, with bigger angle to lift me back to my initial posture?

Thank you

Is the 475 with you on the bike?

Sus forks are expected to compress about 25% of their full travel with the rider in the saddle, so if 475 is the length for unloaded bike, the 440 might be about right for actual riding conditions - if you had your sus fork correctly set up.

Otherwise then I would indeed expect a 35 mm drop to cause some noticeable changes in both rider posture and handling characteristics.
If you are happy with how the bike rides at 475, maybe look for a longer sus corrected fork?
If your bike has disc brakes, you can consider shopping outside your wheel size for a better fit.
But do notice that wheel size does influence the possible load on the fork, and some manufacturers do differentiate between 26" and 29er forks for that reason.

So what do you have now?

Suspension correction generally is for bikes from the 90's, everything available since about 2000 (going on a 26" 80-100mm bike) is corrected out the box.

You really need to provide details of the bike, wheel size, fork/travel now, planned replacement etc, otherwise, it's all guess work, as with the info given, you could have 3 different wheel sizes which would all give different answers.

+1 . have someone else measure the axle to crown race seat distance while You are on the bike, as if you were riding it, and taking that compression sag into account.

Is this a 29?

if so, you'll want a longer fork

+1
This is the important point. Forks settle when you sit on the bike and the dimension you want to match is the settled one.

An easy way to measure is to put a cable tie or twisted pipe cleaner on a stanchion, loose enough to slide, but tight enough to stay put on it's own. Sit on the bike and the slider will be pushed up. Now measure the gap between loaded and unloaded position on the stanchion, and subtract that from the unloaded axle to crown distance. That will give you the target dimension to match.

IMO, anywhere within an inch or so to either side of the target is good enough unless you're an expert bike handler who might feel he difference. Depending on how much drop or raise you end up with you might want to compensate the stem height, but that can wait until you ride it a while. Who knows, you might actually prefer the new height.

Ok thanks guys,

475mm is unloaded, so i guess i will sit on the bike and have a friend measure it.

It's is 26" with v-brakes

What's the fork travel? Most suspension forks are designed for ~20% sag. So a 150mm fork would have a nominal sag of 20-25mm depending on rider preference.

In this case, measuring with you on the bike may only be misleading, since it's likely the sag was never properly set to begin with.

475mm a/c would be a 100mm travel fork
25% sag = 450mm
20% sag = 455mm

These 100mm suspension corrected forks would have a closer a/c measurement to what you'd want.
453mm a/c: https://surlybikes.com/parts/forks/1x1_fork
447mm a/c: https://surlybikes.com/parts/forks/instigator_fork