I chatted with a rep and he said to ignore what was posted and just go in and apply for the rebate and it would work with my legacy plan.
Sure enough, worked without a hitch on my main phone, on my second line I had to jump through a few hoops, but it worked as well
Is you legacy plan the Select Choice or something else? Every time they come up with new plans they chip away at the value.... my legacy "Select Choice" plan offers 1GB more data than the same-priced "Simple Choice" plan, so I'm loathe to change.