SCBG rules, page 22 wrote:
During the active player’s turn, except during a battle, he may voluntarily destroy his own units, bases, and transports
There are no 'player turns' during Regrouping Phase. What's more:
SCBG rules, page 6 wrote:
Active Player: The player who is currently executing one of his orders.
Per above definition there is no Active Player during Regrouping Phase therefore one can't destroy his units during Regrouping Phase.