I was speaking to a Head of English the other day. She is well into her sixties, and I was asking about when she might retire, citing ever changing specifications as a reason form just not having the stomach for it any more.
She said that she was completely unfazed by new specs, as she felt that nothing was ever new. A new spec is simply something that she taught at some other point in her 40+ years.
I think the hardest thing for a head of department is working out how modules work, and tiers. With the move to single tier linear, this pressure will go away. It will be much more down to plain teaching of the specification, and less worried about admin.
For most subjects, a new spec is a slight rearrange on the SOW. In practice, you only have so many resources in your school, and you have your own favourites that you will find a way of incorporating into the SOW. Exam boards publish fairly full SOWs which you can personalise to make them work at your school.
If you are part of a large department, everyone can take one course and produce the SOW for everyone else to use.