"then i'd have to leave the beds empty for 18 months??"

Yeah

I would suggest you consider the change over date to be 1st July, rather than 1st Jan that way you will have to use some catch-crops, rather than moving to next-but-one rotation's crop a year early!

"some people sow peas where the current peas are growing"

Its not a problem until you get disease, or "soil sickness" because the mono-cropping has taken all of a particular mineral out of the soil. But then its hard to fix. So prevention-better-than-cure.

Don't think there is any Right / Wrong way, its a balancing act with the voice of experience (not mine I hasten to add!)