I have not read Tomkins since grad school (many years ago) but Dons reminder of the idea that the ways we learn to handle affect determine our character structure raises the question of whether learning to handle affect differently produces changes in character structure.
Behaviorists have tried to help clients learn to do things differently for ages. Recently a number of cognitive-behavioral approaches have focused on helping clients learn to handle affect differently. I have done a bit of work on this myself but it is done more explicitly in a couple of other approaches.
Marsha Linehans Dialectical Behavior Therapy uses skills training groups in combination with individual cognitive-behavioral therapy. The thing I find most interesting about the group sessions is the selection of skills which are taught. The most important of the skills taught are Mindfulness skills (skills adapted from Zen Buddhism for integrating the emotional mind and the rational mind). The groups also teach Interpersonal Effectiveness skills (which go far beyond assertion), Emotion Regulation skills, and Distress Tolerance skills. I recommend the Skills Training Manual for Treating Borderline Personality Disorder (Linehan, 1993, Guilford) if you are interested.
A different, but complimentary skills training approach to borderline personality disorder is: Farrell, J.M. & Shaw, I.A. 1994. Emotional awareness training: A prerequisite to effective cognitive-behavioral treatment of borderline personality disorder. Cognitive and Behavioral Practice, 1, 71-91. This approach starts with training in Emotional Awareness followed by training in Distress Reduction, and Emotion Regulation and then work on identifying and challenging maladaptive schemas.
If it is true that learning to handle affect differently changes character structure, then behaviorists may have found a way to change personality.
Of course, it remains to be seen if these techniques really produce personality changes and whether they have any advantages over other therapeutic approaches. However, I think they are quite promising.