Simple:
while (Analog x-inches + HD-decoder <= Digital (x+5)-inches) {
; // do nothing
}
The price of the external decoders will decide everything. If they are cheap then there isn't going to be any mass exodus. If they are expensive there will be a brief deprecation while the asian markets get their butts into gear, when the price will stabilise as people start re-buying all the analog TVs going around under-priced until they end up the same again.

Of course, this isn't even going to start until six days before the analog system shuts down, what with people being the way they are.
