Add compost every time you plant something new.

I just posted this question too, i guess i should have looked further down,lol. Im going to do a small top up with fresh Mel's Mix but my compost pile is being a slow poke so I dug treches through the beds and hucked in the unfinished compost. Hopefully the worms will do the job :)

It seems to me that in Mel's Mix, compost is the only perishable item needing replacement after a year, so if you replace with more of Mel's Mix, you would actually be altering the basic percentage of ingredients. (Which is fine, but realize that is happening).
Vermiculite is a mineral so should not decompose at all, and peat lasts a long time.