The rules don't say otherwise, so yes, they can reuse their robot. OPINION: If most/all of the team members are returning, then by all means let them re-use their code and their 'bot if they want to, so they can build on what they learned last season.
The rules don't say otherwise, so there's no problem with starting with a robot from EV3Lessons (or anywhere else), nor is any special consideration given in judging either pro or con. The important thing is that they can explain their design to the judges; why they chose to start with an existing design.