Primarily this appears to be a bad word: to regress means to slip back, most likely beyond your control. If you are looking for a word for the opposite you might plump for 'aggression' but you might ask, is that not bad itself? Regressing to aggression seems not to have much going for it. But is that right? It's a strong word aggression, but when you think of it in terms of the child it could be used to signify an attempt to have innate needs, such as hunger, attended to when it seems to be being ignored. So aggression becomes for some a necessary survival mechanism. It is more like negative and destructive if a withholding has been experienced. Otherwise it is a form of affirmation.
A better word for the opposite of regression is progression. The reason for stressing these opposites is that they are, it seems, integral for growth of the psyche. In this context then, Regression means a stage in a process of development of returning to an earlier one, a place of origin, to the fundamentals of creative reorganisation, a call as it were from our internal child. The paradox is that a seeming deterioration is vital to deeper development towards a capacity for play and intimacy. It is a challenge for both patient and therapist.