This is a biggie... worked out by Drew in Perth
... however I have found a way to fix this issue:
1) Select the column.
2) Select edit Family and open the family for editing.
3) select "Settings" on the top menu bar, then select "Family Categories and Parameters"
4) There is a check box at the bottom that reads "Automatically joins geometry to walls" - Uncheck this box.
5) You can then change the family category to structural from this same dialogue box.
6) Reload the column into your project and viola. All columns of that type upgrade to structural columns and "detach" themselves from the walls etc.
Hope this makes sense.