We have performed the experimental and numerical studies exploring dynamical processes in a harmonically mode-locked fiber laser responsible for birth and annihilation of solitons in the laser ring cavity. The injection of external continuous wave (CW) allows one-by-one change of the number of solitons in the laser cavity thus enabling the fine-tuning of the pulse repetition rate. We present new experimental observations of the laser transition dynamics associated with the changes of the soliton numbers and give clear insight into the possible physical mechanisms responsible for these effects.