Now that we have branch insignias to go along with dog tags, I see a good amount of people served in more than 1 branch, and a couple who have served in 3 or 4! I'm just curious why people decide to change branches and what that entails. And thank you for your service, to all of you.

I was facing the draft in 1968 after I ran out of money for college. The Air Force had a 9 month wait for enlistments and the Navy had a 6 month wait. I scored high enough on my tests that the Navy recruiter had a cancellation a few days later. I took it. No idea of what rate (MOS) I would get. Turns out that they were not taking what I wanted that month so I ended up a Communications Tech. Morse code school first and more technical school later. I had good duty stations but I got out to finish college. After college I taught HS and Jr HS for a year. I had joined the National Guard while I was in college after the Navy so I got a taste of the Army. I then decided to reenlist in the Navy. However, the Navy was going to take a stripe and only allow me back in my old job. The Army also took a stripe but I was able to choose my MOS and duty station. So it turns out Navy 4 years, Army 16 years. I taught for 10 years after I retired from the Army.
 Since I was in the Army Guard I was just transferred to the active Army. When you joined the guard you got your old rank and no basic, no matter what branch you served in.

Why I went from the Corps to the AF. I was one of a very few paint qualified (aircraft) guys at the time. I was told that If I reenlisted I would continue painting. Everything about the paining and haz-mat job were making me sick. Med record got real thick real fast.
Joining the AF was as easy as walking into the recruiters office a couple months after my EAS.
I left the AF for cultural reason. It just wasnt a good fit for me.
Joined the Navy as a SeaBee. Good fit for me. That was just as easy as joining the AF, except I joinned Navy before I was out of the AF, so I had to get a release from the AF.

A lot of guys go from the Corps to the Army or other branches. Once you do basic in the corps you can go to any other service without having to do their basic. It is easy to get burned out in the Corps, especially infantry. Different stokes for different folks. The grass is always greener on the side, etc...

From time to time one might get tired of living in a fox hole and look up at the Air Force guys flying over going back to their warm meal and dry comfy bed or look out to sea and realize that in the Navy you life is always the same warm bed warm meal PX movie ...
Or someone just gets tired of the BS  (like in any job) ....  quits.... thinks about it a while ......and decides to join the Guard or Reserve to complete enough time to get certain benefits.  Or some branch offers an incentive $$$ for a job you are qualified and educated to do.

I switched from Army reserves to Air National guard on a conditional release.

First step is talking to the recruiter at the branch you want to switch to. Then you gotta convince your unit to sign the papers.

Worked for me.

Not me, but a guy I worked with retired a few years ago as a SSG on active duty. He had 32 years of service starting as a Marine, then Marine Reserve, Army Reserve, Army Guard and finally Active Army. His ERB was interesting to look at.

I did 8 years in the Corps. 3043 for 7 years and then got orders to drill field, oh, wait we need recruiters worse, that's where you are going. 14 months later I left. I was in the top 1% and they didn't believe I was leaving until almost the end. Anybody can be replaced, lol. Recruiting sucked.

I joined the ARNG in Indiana. Had better training in that unit than I did the entire time I was in the Corps. 1st Desert Storm was going on, we were going to more than likely deploy. Nope, over too quick, no need.  Saw a Coast Guard commercial at 3am. Had to do most of my own paperwork to get in, lol. Had to go to 6 weeks at Cape May, was a vacation away from the wife, lol.  Did everything from boarding to search and rescue on the great lakes. Went to Yeoman school and took a cushy job.  Ended up working with the Port Security Units and standing up the PSU training detachment in Port Clinton. Now it is at Lejeune and a joint training center. Took a transfer to Topeka to work in the pay center and retired here.

To be honest they all had their good and bad things. I have three dress uniforms in the closet from three different branches, lol.

Back in the late 80s I went from the Army National Guard to active duty USAF.

I was a tank crewman (19E at the time) in the National Guard and E-5.  All I really wanted to do was go active duty army, stay in tanks and see how much rank I could hold onto.  The best the army recruiter could do for me at the time was a reduction in rank to E-3 and an enlistment "open general" ...  pretty much any MOS the army needed at the time.

The air force recruiter was willing to bring me in E-3 and whatever job skill my test scores opened up for me.  So I retook the ASVAB, qualified for one of the best jobs I could ever imagine and ended up doing 24 years in the air force.

Sometimes things just work out.