In current versions, you have to close the Workshop to let authors read the assessments. The Workshop has to stay closed for a while to give them chance to read the reviews. Then you can switch back to the submission phase to start another round. Neither assessments nor allocations are lost. However, there is no versioning. So assessments filled during the first round will be still considered as valid. For that reason, you may want to clear all filled assessments forms before switching to the assessment phase in the second round. Unfortunately, assessments from the first round would be lost forever then.
There is a plan to improve the overall workflow in the future to solve this and other issues that teachers have in real scenarios (like the ability to start assessing without the need to wait for the whole class to finish their submissions etc).