For Issues and Tasks, you can create a report (either an assignment report or two separate reports - a task and a issue report) with filtering to pull in the tasks/issues that are assigned to a user (or a job role). From there, you can select them all and do the changes to them them all at once.
For projects, basically the same thing except you'll need a project report and filtering for the owner to equal the user(s) or job role you want to reassign from.