Not sure what you mean. Hires can definitely be toggled mid-frame with no flickering. It just varies how long each color is fed to the display.
Same for pseudo-hires. It's quite possible to use two BG layers in mode 0 to make a 512x480 screen =)
Interlace on the other hand, while it can be toggled at any time and takes immediate effect on PPU addressing, obviously cannot take effect immediately on the TV. It caches the interlace setting somewhere and will apply it consistently to the entire screen. Not sure if it does it every single frame or every other frame, though. Too hard to tell.
