Dragging + dropping to allocate on the roster makes it easy to re-allocate carers on-the-fly.
By default, the carer's default contract will be used for the new booking. However, you can quickly change the contract before saving.
Once you've moved the carer's booking on the roster, click Preview & Save.
You'll see each change is listed; for re-allocations you'll see a line along the lines of:
"Carer changed from Carer A to Carer B with default contract".
Click default contract to switch the contract for that allocation.
Once you're ready, click Save to allocate the carer.