I think the belts can be changed. It is easier on the Buick because the H body Lesabre was all pillar belts in 1986, and the C body Electra was all pillar belts 1985-1988. (USA) Unfortunately the H body Bonneville was introduced in 1987 when the door mounted belts began, so it can only be done with parts from a Canadian model.
For my Buick, all I need to do is get the proper belts and trim panels from a 1986 Lesabre or 1985-1988 Electra. I have checked, and the upper anchor exists in the B pillar, with the required reinforcement, it just needs to be threaded. The lower anchor point I have not checked for yet.