I have a feeling that when designing the workshop module, the phase division was not an easy task. The existing phases seem not sufficient to meet the the controler's (normally teachers) needs in a peer assessment procedure.
Ideally, when a peer assessment scenario is well-developed, if students are familiar with such peer assessment process, the controler will just spend time on setting up assignments, assessment elements and a number of parameter values including the date/time for each stage. The controler does not have to "switch" the phases but just monitor the whole process till the final grades come out. He can also adjust the parameters anytime if he wants.
I am not sure of the aim of current workshop module development. If the above scenario is taken into account, I would think if it is better to set up the following parameters to replace the phase switch.
1. Assignment submitted between: date/time - date/time.
2. Assessment submitted between: date/time - date/time.
3. Assignment resubmitted between: date/time-date/time. (Optional)
In this way, the phases can be defined by the controler. For example, the controler can decide whether the students can assess right after they submit, or after all assignments are submited.