Can I convert all my polygons to boxes? (moving from Visio to Dia)

[Aleksey Tsalolikhin <atsaloli tech gmail com>]

Hi.  I exported my network diagram from Visio to XML, and then
imported it into Dia.

My rectangles (that I used to represent objects like servers,
switches, firewalls, etc)
appear in Dia as polygons, which means I can't easily resize them by grabbing
and moving one corner; I have to move each corner individually, and even though
it looks like there are four corners, actually there are 5, so it's
messy to resize the boxes.

I see there is a "Box (R)" object, which is what I want - is there a
way to automatically
convert the Polygons into Box (R) objects?

Thanks,
Aleksey