For a resolution change, couldn't you just scale the old last frame to the new resolution and use that as the basis for more P frames? (originally replied to the wrong comment)
Longer answer: The codec needs to support it. Codecs actually allow prediction from multiple reference frames, and maintain a buffer of them (2 to 16, depending on the codec, profile, and level). An individual frame may refer to several (potentially all) of those. So re-scaling up to 16 frames for every frame you decode will get quite expensive, not to mention the generational losses of doing this repeatedly for every resolution change. In practice what happens is you scale individual blocks when they get referenced by the current frame. But that has to be integrated into the motion compensation routines of the codec.
Longer answer: The codec needs to support it. Codecs actually allow prediction from multiple reference frames, and maintain a buffer of them (2 to 16, depending on the codec, profile, and level). An individual frame may refer to several (potentially all) of those. So re-scaling up to 16 frames for every frame you decode will get quite expensive, not to mention the generational losses of doing this repeatedly for every resolution change. In practice what happens is you scale individual blocks when they get referenced by the current frame. But that has to be integrated into the motion compensation routines of the codec.
Both VP9 and AV1 support this, for example.