Hi Ye,
Try to create a seperate query and fetch Team Lead and Assigne in it.
Then drag the merged dimension of Assignee into your report block. You can also put a hierarchy as Team Lead --> Assignee at universe level.
Regarding your requirement related to change in Team Lead, i think it should be handled at database level.
You can either put a new column in the table as Indicator and can put the value as 'Y' for new team lead and 'N' for older one. You can put the report query filter as Indicator = 'Y'.
Regards,
Yuvraj
