Most DVRs are capable of playing MPEG-4 files from internal and external storage devices. That is completely separate from what a DVR's tuner is able to capture. All ATSC tuners expect MPEG-2 broadcasts. They won't be able to record anymore if the format changes to MPEG-4, thus rendering current DVRs useless, just like current TVs. The TViX and VRX are also capable of playing the MPEG-4 format, but like the CM7400, they can't tune or record it.
Well, the A/72 addition to the ATSC standard for AVC support was supposedly made in 2008, but I can't find either confirmation or denial of whether it's integrated into all ATSC tuners. Since the update was made before the transition, it seems reasonable to assume that all post-transition sets support it, which would indicate that all DVRs and TVs with ATSC tuners would be unaffected by a transition to MPEG-4 broadcasting, and that such a transition could be made whenever the broadcasters wanted to make it, since it wouldn't require hardware upgrades on the part of viewers. If that's the case, the VRX would probably already be in good shape, as would the TViX and CM7400. Perhaps someone with more knowledge of the standards could clarify this point further.
As for ATSC 3.0, that's much further from completion. It's based on HEVC (H.265), which is the proposed successor to H.264/MPEG-4 AVC, which is the successor to the MPEG-2 system currently in use for ATSC broadcasts. The HEVC format hasn't even been finalized and approved yet (it should be in 2013), so it won't be available for a while, especially for OTA TV.