In the sense that one second is 9,192,631,770 transitions between two energy levels of the caesium-133 atom, no, there's no change. 50 billion years is just shorthand for a multiple of this number of transitions. International Atomic Time, or TAI, is precisely defined for this kind of measurement.
Whether TAI is a fixed number of leap seconds away - potentially billions - from UTC, is another question entirely. If 'one day' is one rotation of the planet Earth around its axis, and 'one year' is one rotation around the Sun, and hours, minutes, and seconds are fractions of these rotations, yeah, those cease to have much meaning when the planet is swallowed up by and dissolved into the Red Giant Sun's expanding chromosphere.
Whether TAI is a fixed number of leap seconds away - potentially billions - from UTC, is another question entirely. If 'one day' is one rotation of the planet Earth around its axis, and 'one year' is one rotation around the Sun, and hours, minutes, and seconds are fractions of these rotations, yeah, those cease to have much meaning when the planet is swallowed up by and dissolved into the Red Giant Sun's expanding chromosphere.