Is their a bowler in the cricket history who has increased his pace significantly so that before he was a medium pacer and then called a fast bowler in international circuit?
Yes, many bowlers at starting carrier bowl with very low pace in cricket club matches. After putting too much efforts and daily practices made them speedy/pacer bowler. One example is like Wahab Riaz Pakistani player (left hand fast bowler)in the beginning bowl with average of 120 to 130 Kph. After polishing them self ,bowl with 140 to 150 kph in international matches.