Do not set yourself up for problems trying to ask for any back rent. You have most likely waived your right to it anyway. If you recall, they have lived right above you for six months without you knowing it. Just stick with the rent increase, remember your tenants are disabled and will get a free lawyer if needed. Ask for $150.00 additional per month if they sign a new lease right away. That would bring the rent total to $850 per month. Don't be greedy, be a savvy landlord.