I have to say that I'm no fan of growbags for tomatoes. I plant my tomatoes directly in the soil inside the greenhouse and renew the soil after a few years. You could also use large flowerpots or even the metal containers from the chippy with drainage holes. As the growbags are enclosed in plastic then I would imagine that they would be a breeding ground for moulds and viruses. So perhaps fresh compost in containers or new growbags would be the way to go and distribute the old compost over the garden beds.

I agree. Tomatoes are heavy feeders. There wouldn't be much nutrition left for the new plants to thrive. Look up 'crop rotation' to see why new compost is the best way to go. If you cannot plant them in the ground, use the biggest tub you can find even a 50 gallon trashcan is better than those little growbags.

I've re-used compost in bags for tomatoes before & found it worked perfectly well. I cut off the short side of 70 litre bags & stood them upright against a South facing wall.
Then I got my arm right in to loosen the soil & evenly mix in a handful of growmore pellets, lawn clippings might also help. Epsom salts might be worth researching as an additive & I'm completely convinced that aspirin sprays on the young plants help combat late season moulds.
If things don't seem to be going as well as they should you can always use seaweed tonics or even miracle gro.
Nearly forgot... Oven dried & very finely crushed egg shells for calcium too!