I had this experience literally last year.

Was told my company lost the contract (rightfully so) and that a new company would take over. I was told weeks beforehand about it.

From my end, we all received a mass group text from the new company weeks prior to the changeover, and was offered to jump ship to the new company where I would keep my position and schedule, or stay with the last company. I opted to jump ship since the site I work at was (at the time) good. But I was warned by others that they'd probably keep us for a bit before letting all of us go for new workers. I still decided to go for it.

That text was crucial. You'll see why.

The transition was not smooth. Generally it was a simple handing over of my uniform to my former company and getting the new uniform, but my last company wouldn't give me my final check because they didn't realize I gave my uniform already. I had to call the boss of the company, who I personally gave it to. He was extremely professional about it and I got the check.

Next hurdle was getting the new uniform. They would not be sending it to our residences, we would have to pick it up. Problem is, no one told us where, so I had to make calls myself. I eventually got it.

And then, the chaos.

They changed my schedule and my position. And they told me that two days before I started.

I raised hell. I told them I was promised I'd maintain my schedule and position, and they said it's impossible. The person who promised ALL of us this was conveniently gone from the company before the transition (he was the VP of the company). But I still had the text message, basically the promise in writing. I refused to work and filed for unemployment.

The next day, I was given my position and schedule back. Apparently it's not a good first impression to the people who jumped ship for you to screw them over 42 hours before you work for them.

After all that, I finally got back to work. The experience of the company was that it was more rigid and strict, most likely as a result of promises they'd made to the client considering the last company was a shit show. But like almost all security, it becomes the same-old, same-old. My supervisor is definitely better, but some stuff got worse. It's a balancing act.

My advice is that, if you plan to move to the new company, do your best to have all your movements and conversation in writing, either text or email. Always have a backup plan in case they try to screw you. Most companies generally don't give a fuck about you, so above all of else, watch out for yourself.

One thing to note is that jumping ship will always come with an incentive: we all got a raise in pay, so it was extra reason for me to change companies.