I am having the same issue as detailed in the post https://lucidchart.zendesk.com/hc/en-us/community/posts/360077453331-Stop-connected-lines-moving-when-extending-a-line and wanted you to know that I am also hitting this a lot. We have a lot of block diagrams connected with lines and often need to change the size of the block to be able to add or subtract some areas.
However when we resize the object all the connections are stretch and usually ends up with overlapping lines and generally unusable. It take a long time to adjust again.
Knowing that the solution is to first disconnect the lines is not going to work as this is as much effort as fixing the lines after they are stretched. This is likely enough for me to start looking for alternative solutions as I'm hitting this all the time. Not a good design.