Legacy model Tablo DVRs are still supported by Tablo firmware and you can still download their apps on compatible devices.
Eventually, legacy Tablo DVRs will be able to run 4th-generation Tablo apps. At such time you'll want to take a look at this article before you make the switch.
If your Tablo is running a very old firmware version, you may need to process several firmware updates to ensure optimal functionality and compatibility with current systems.