I've transplanted in flower before with no problem. Most people don't recommend it because newbies don't know how to transplant properly.
There's a chance that placing them into new soil will make them focus on root growth though. IMO the main risk is that the soil they are transplanted into has high nitrogen. If you're an experienced gardener (which by the looks of those trees, you are), then you can likely get away with no transplant shock.
Is that grass heavily fertilized with nitrogen? It's quite green.