I did 8 years secondary, 6 years primary and have gone back to secondary.
It is in NO WAY easier and it fairly broke me. Going back to secondary has been a great relief.
Behavior is not easier to manage. This is because you will have a lot more SEND and EBD students who will not yet have ECHPs and 99%of them won't go to special ed until year 7. In my last year 6 class I had 34 kids and 7 of them were SEND. All of them working at year 2 or below. Without a class TA because it is VERY rare to have one now. And i've had year 6's call me a cunt and attack me with a chair.
Progress is far more scrutinised because the cohorts are so small. Once SEND are discounted, every child counts and you will be expected to do before and after school interventions to make them hit targets for no extra pay.
Whilst the children can be delightful there is less joy in the curriculum than there was and the days of topic learning are gone. You can expect maths, english and more of the same.
You will have to lead a subject without a TLR. Budgets are far more screwed than secondary so don't expect UPS. I have found primaries are less unionised and far more likely to expect crazy working hours. I used to get in at 7.30 and be the last one in. You will have to prepare the work for the cover in your ppa time. Leadership is insane and I found it bonkers scrutiny wise. And the marking is uttterly crackers. Every book, every night.
And you see a LOT more of parents. You can be nabbed in the playground. And it's your fault they lost their jumper.
And I never had a lunch that wasn't taken up by sorting out arguments, social media (it starts in year 5 or before!) or fights.
I find my marking load much more reasonable now and the school I am in is unionised- meetings don't run over and nobody gets in stupidly early or stays stupidly late. And the behaviour is actually better because there are sanctions- something lacking in my primary experience. Not that it's perfect- far from it! All I know is i've got my weekend back a bit and am less scrutinised.
In terms of how to swap- just turn up! There are such big staff shortages that as long as you are willing to learn they'll have you. I'd look at the ks1 and ks2 sats and get up to speed plus if you can ask to shadow a primary teacher for a day. My secondary is all through and a couple of teachers do year 5-8 maths.