Going from 75mm fork to 100mm?
 

Would this be alright to go from a 75mm fork to a 100mm fork, any opinions?

yes

It depends. 75 to 100 is more noticable, than say, 200 to 220. The bikes geometry will be changed significantly.

What's the current head tube angle?

What's the current bottom bracket height?

What's the current frame size and wheelbase?

Without knowing this, I'd recommend 80mm as well. The extra 10mm of axle to crown height difference between my 75mm Jett XC and my 80mm Duke XC was noticeable in its effect on handling.

If I went from 75mm to 100mm on my Backwoods (old school geometry), it would get hyper stable and downhilling would be a blast but I could no longer negotiate tight switchbacks.

My Zaskar's head tube is about a degree steeper (newer school geometry) and the effect would be similar but tight switchbacks wouldn't present as much of a problem.

You don't necessarily need more travel. A better quality fork will provide a better ride. That is to say, if you're bottoming out at 75mm and (for sake of argument) you replace it with a fox f-80, you probably won't ever bottom out again, if the fork is set up correctly.

Interesting, I asked almost the same question on the mechanics forum a few months ago and got the completely opposite responses. I proposed changing an 80mm Suntour shock on a 2001 Hard Rock frame up build for a 100mm Manitou Axle I had hanging around. The responses were unanimous that it would not be an issue.
I made the change and took the bike out for a shake down ride on the street, it handled fine. As soon as my shoulder heals, I'll try a trail, but I expect it will work out fine.

I wouldn't be in the unanimous but more in the it will probably work camp. If I'm not mistaken, my Backwoods originally had a 70.5º head tube and a 75mm fork with a ~440mm axle to crown height. I'm now running an 80mm fork with a ~450mm axle to crown height which slackened the head tube to 70º. Going to a 100mm fork with a ~470mm axle to crown height would slacken the head tube all the way to 69º.

This is merely an example of a situation where it wouldn't work as well as hoped. The trails around here are riddled with tight switchbacks and negotiating them efficiently is piece of cake on my Zaskar but currently requires much more careful planning on the Backwoods.

Taking another degree of the head tube and adding another 6mm to the wheelbase as well as another 6mm to the bottom bracket height would have the Backwoods handling more like a freeride hardtail. If that's what the OP wants, I say do it. However, around here it would have more of a tendency to wander on all the extended granny gear climbs and having to swing another 6 or more inches wider around some of the aforementioned switchbacks would likely put me off edge of the trail and into the trees.

NO and especially if it is an older frame. Go with what the bike was originally designed for. I went from a 75mm fork to a 100 and it rode like a chopper. I then went to an 80mm fork but the damage was done. I cracked the head tube. Game over for the frame.