Haven't done it myself (I've always been primary) but have colleagues and friends who have and they all said how hard it was to go down rather than up. They totally underestimated the constant stress and interaction with younger children. They also said that they didn't appreciate that (apart from PPA) you get no free periods and that having to think really carefully about how you talk to younger pupils. You can't assume prior knowledge with even very simple thi Gs that they hadn't considered. Having said all this, not one of them regretted the decision to move down age groups and were so much happier.
What is your subject? Would you consider the private sector? I have moved from secondary into primary (prep school) but I teach my subject only. However now that I am in the primary sector the school are happy to help me gain experience in other subject areas and send me on courses to update my knowledge and make it KS2 specific. I'm not sure I would get a job teaching primary in the state sector as my Education qualification is still secondary specific - but to be honest I am fine with that!
I did a Secondary Science PGCE and then did my NQT year in Y4 without skipping a beat. However, that was 10+ years ago when there was a desperate shortage of Primary teachers and the first school I applied to literally bit my arm off.
There is definitely no requirement to retrain. The problem is that, now, there seem to be far more Primary teachers than jobs (not in my area TBH, but in most places) so you would be competing with cheaper people who were Primary trained. Can you afford to go on supply? That would be a way to get Primary experience and get yourself known to some schools.
I really want to change as I've been in secondary for 10 years & feel I made a mistake doing secondary rather then primary. Any experiences, how did you do it. I would like to do the pgce again but I know this is probably unnecessary.