I heard the same, but the person who contacted me said that they are not beginning the conversion until Opening Day as they are now testing out the new system for the 2018 games. He said it could be fast or could take a while. He even mentioned a possibility of completion around the all-star break, but said he had no way to tell for sure.

I had them refund my 2018 subscription. I only use MLBTV for photomatching and the price typically reduces from $115.99 to $29.99 around June, so I'll probably just purchase then.