Wait to redo veneers
if they perform crown lengthening, i suggest you should re do the veneers at least 1 month after, so the healing comes properly, this way they would look more even and natural, hope this helps.
Crown Lengthening to improve smile
There is little question that well done crown lengthening will improve your smile. The is a lack of symetry between you front teeth at the gingiva.
The best time to crown lengthen is before veneers are placed. Doing crown lengthening after will expose veneer margin (where tooth meets veneer)