There are a number of ways of modifying a curve to have different end points (e.g. different distance or direction) while retaining the "same" curviness. Different transforms do the right or wrong thing for different cases.
Here's the behavior I would love to see (not saying it's the best, but it's what I often do manually in diagrams):
1 Preserve the user-set position of the attachment points relative to the bboxes of the connected objects (in % of width/height of a connected object), allow it to be changed manually by dragging the endnode of the connector; make it at 50% (center of one of the four sides) by default. Rationale: connectors going to corners rarely look good, especially with rects.
2 Preserve the direction of the bezier handles at the connector's endnodes. That is, if an endnode runs into the object at 90 degrees, preserve that angle no matter how the object is moved.
3 Scale the length of the bezier handles at the connector's endnodes based on the distance between connected objects (more precisely their attachment points). That is, if objects become 2 times closer, the handles become 2 times shorter.
4 If the connector has midnodes, their positions and bezier handles must be simply scaled proportionally in both dimensions. E.g. if the startnode and endnode are on the same vertical, then the horizontal projection of all bezier handles of midnodes is zeroed.
Variation: when the relative position of the connected objects flips in one of the dimensions, you can flip the corresponding connector: that is, move it to the opposide side of the bbox and flip the bezier handles correspondingly.
This behavior could be implemented as one of the possible "styles" of connections, with a way for the user to switch styles as needed. Your current behavior could be another style. I attach a manual example of how the above described style could look in a diagram.
