Greenhouse Care

Assuming you mean in beds in the greenhouse (i.e not containers) then I change mine every year, I've found that tomatoes don't do at all well grown in the same soil twice. I use a mixture of soil taken from another part of the veg patch mixed with lots of my home made compost. They won't do very well in straight peat, you risk getting trace element deficency problems especially with the tomatoes, you'd be better off using a mix of 50/50 peat and soil.