If they did the old raids up to current light level, those raids would only go to 300-310 gear. And only AFTER majority of the population has finished normal Oryx. The reason behind this is if you have 4 raids all giving you gear at 300-310 light level, you will be 310 within 2 weeks (8 completed raids). This would then trivialize the current newly released raid and make progressing through Hardmode that much easier. Edit: 1 day to 310 if you had 3 guardians. They want raids to last as long as they can, however, once they have run their course and served their purpose, they have to be left behind. Newton's third law. Every action has a reaction. New raid comes in, old raid goes out. It would kill the difficult aspect of raids to continuosly update the old raids to the current normal raid light level. Between expansion it might be OK. 2 months before a new expansion, releasing old content to whatever the current hardmode is set to might be OK.