I had read that this would still work even if you prepaid for the service. The article stated that the prepaid portion would be "paused" by disney until the free year was up. I am not sure how accurate that is, I am still looking into it.
As it turns out, those who got in early on Disney Plus will still be eligible for the Verizon promotion, if they’re on a qualifying service. According to Verizon, customers who have already preordered Disney Plus will have their existing subscription “paused” for the 12-month period. The subscriptions they already paid for will then resume through Disney.
Under Verizon’s special promo, Disney Plus will be free for one year to 4G LTE and 5G wireless customers on unlimited-data plans as well as new Fios broadband or 5G home wireless internet customers. After the one-year promo pricing expires, Verizon customers will revert to the regular $6.99 monthly subscription price
Business terms of Verizon’s free one-year giveaway of Disney Plus aren’t being disclosed, but it is assumed that the telco is paying some per-subscriber amount to the Mouse House under the pact. Verizon says it has about 100 million total wireless customers and that about half (50 million) are on unlimited plans that are eligible for the free Disney Plus offer.