What happens if I push a pawn to the 8th rank and press the clock before promoting? What happens if I push a pawn to the 8th rank and press the clock before converting it to another piece?

Does it make a difference if it's Blitz or not?
"If the player has moved a pawn to the furthest distant rank, pressed the clock, but not replaced the pawn with a new piece, the move is illegal."
Source: Fide Laws of Chess taking effect from 1 January 2018 at 7.5.2.

If a move is illegal, it's illegal in Blitz, too. What can differ between Blitz and classical is what the penalty for an illegal move is.

Previously, in blitz/rapid a player making an illegal move would instantly lose the game.

However, January 1, 2018 revisions to FIDE Laws of Chess acted "to equalize the rules for classical chess on the one hand, and rapid and blitz on the other, with regard to the penalty for making an illegal move."


  Before, in both rapid or blitz an "illegal move" (which includes moving with two hands, or not making a move and pressing the clock) would instantly lose the game in case the arbiter intervened or the opponent stopped the clock. Now, in the first instance it will lead to a time penalty and only if a player does it again, he or she loses—just like in classical chess.


Source for quotes on 1/1/2018 change: Peter Doggers, New FIDE Laws Of Chess For Blitz, Rapid Still Not Perfect