Dish has always had a plan to replace receivers as they become out dated in the past. Believe me they do not want to lose customers because of a conversion. The only real issue in the past has been that owned receivers can be replaced with leased receivers. Some have complained hard enough to get owned replacements in the past, but usually it is leased.
Essentially you still own your old receiver but it is just a fancy door stop, so you really do not lose anything by having a replacement being leased (since now you can avoid DVR fees with the 211s if that is your goal). There is no difference in fees between owned and leased. Dish has also replace them without extending any commitments.
So, all in all Dish has worked hard in the past to take care of their customers when they change technology.