Since Kevin asked the question, I decided to look both in my FAA files and my picture collection and see what I could figure out.
Here is more information than you probably care about.
It appears that a total of 23 Helios have been converted from round windows to the square windows or more accurately trapezoidal windows, and 5 have had the rear round window completely removed with 5th and sixth seats removed.
19 have paperwork showing the change and 4 don't have FAA paperwork, possibly logbook entry but no way to know.
No H-391's have been converted to round windows, not sure but I don't believe there is any difference in the cage that would keep it from being done.
Only 3 H-395s and have been converted and on 1 H-395 the trapezoidal window was later removed and converted to an enlarged baggage area. 3 H-395s have no rear windows 1 an H-395A was ordered from the factory with no rear windows.
17 H-295s have been converted to the trapezoidal windows and 2 have had the rear windows and seats removed for a larger baggage area.
Only 3 H-250s have been converted to the trapezoidal windows.