When I was a kid we moved 3 times, always during the summer break. The 1st was between 4th and 5th grade, then between 8th and 9th, and between 11th and 12th. The 1st move was from a mid-sized town to a tiny town and the big problem was the quality of the new school system. It was a dirt-poor district and the teachers fell into two categories - about to retire and didn’t give a shit, or fresh out of college but not good enough to get a job in a bigger district. Did I mention it was tiny? If I’d stayed there the graduating class of 1980 would have been 12 kids.
Like Manda Jo, I went to eight different schools as a kid.
And for me…it depends.
Moves where I had a lot of friends and roots were very traumatic to leave. Especially where distance was involved and the whole “we will visit” thing was not really possible.
Places we were in briefly, no big.
Where we landed also helped. Sometimes I’d be welcomed - sometimes I made an effort to fit in - sometimes I was the strange new kid and never did fit in. And whatever advice you give your kids “be yourself and people will like you” is not good advice for middle school! My most successful transitions were the ones where I wasn’t myself and I mimicked the behavior of those around me - and behavior I’d learned at other schools was acceptable.
Don’t underestimate the trauma - just because you can’t remember the name of your best friend in second grade doesn’t mean it isn’t a big deal for your kids to leave them behind. Even if you know that it was a big deal for me and now 40 years later its like “Rob…no…Scott…no…gah, what was that kid’s name!” It WAS a big deal at the time.
We moved when I was 5. I don’t remember being upset about it. We moved just a couple hours away, and had relatives in the area we moved from, so we would visit them and a family who became lifelong friends every year or so, and that gradually tapered off.
The move I’m considering would be from a town with about 35,000 people to a suburb of a city with 600,000 people. It would be in the same state though, only about 100 miles away from where we live now. We would still be able to visit family and old friends. I know my daughter (10 yo) will think that her “life is over” as that’s one of her favorite sayings. 
I haven’t really discussed this with them yet, I’m not really sure if I even want to make the move.
I moved a few times, from elementary school age (third grade or so?) up until late junior high. Each time I met it with mixed feelings; I was excited about going some place new, but also feared I would miss my old friends, and that it would be difficult to make new ones.
When I was eight, my family moved from San Francisco to the burbs. It was very traumatic for me. I remember spending the night at a friend’s house shortly before the move and just crying the whole time.
They should be fine. When I was 8 and my bro was 5, we moved from Colorado (where we had lived for our whole lives) to Michigan. It was just one big adventure. Our parents were exited, and so were we. I agree that transferring to a new school is much easier at the beginning of the year, since it’s new for everyone and not just your kids. Just be there for them, to alleviate any fears they may have and set the example that it’s not a bad thing to move. Good luck! =)
Where you’re moving from and too is a big factor. I also think your kids’ personalities have a lot to do with how hard or easy a move will be on them. My son has lived in four states, and we’ve moved a few times within communities. Our most recent move was specifically to get him into a better middle school, but in the same district, so there are kids from his elementary school there.
My own son is not particularly outgoing or involved on teams or activities. I think he’d adapt to a move more easily than someone his age who had strong ties to friends and school and a gymnastics or dance or martial arts studio. As long as we moved to a place where there are other boys his age who like to skateboard he’d do pretty well.
Sure you will, at first. When we moved the last time my parents promised that they’d bring us to visit old friends, but after less than a year they realized that a 140 mile round trip (even shorter than you’re talking about) was a big pain in the ass, so I lost touch with friends I’d known since the age of 3 
My brother and I moved from England to Australia at the ages of 7 and 9, respectively. That was at a time when long-distance travel was by ship, so visiting old friends would have required a two-month round trip. I don’t remember either of us being traumatised by the experience.
I think a lot will depend on the nature of your kids. I’m the kind of person who doesn’t keep up with old friendships and doesn’t place a lot of value on familiarity and not changing; moving didn’t bother me. My husband keeps up with old friendships from childhood, and hates change; moving killed him.
I moved 4 times in my childhood, across or to different continents every time. I hated it. My brothers hated it too. When I had kids, I swore I would never do it to them. Recently we were forced to move across town, and my primary-schooler had to change schools. She is settling in slowly, as I thought she would, as even having a new teacher in a new grade took her a while to get used to. Unlike my brothers and me, I don’t think she was actually traumatised by the move. Everyone is different.
This was very true for my parents, but not at all true for me (I was 7 when we moved to our tiny town).
I was 5 when we moved from Kansas to NYC - it hardly affected me at all. I was sad to leave my preschool friends and then instantly forgot about them and made new ones. I remember parts of the move (we drove cross country in a truck) as a strange and otherworldly adventure.
My brother was 8 and I think it was a bit tougher on him leaving his friends. However I don’t believe he would describe himself as “traumatized” by the experience. It isn’t something he talks about having any significant effect on his life, beyond “I missed so & so for a little while.”
I moved from Britain to Bahrain when I was 4. That was fun. Moved back to Britain when I was 6, which was less fun, because my parents were divorcing. Moved from the southwest to the middle of England when I was 10. That was okay. Moved to Florida when I was 13. That was awesome.
They’ll be fine.