I was able to do everything I tried--resize walls, change text, draw new walls, etc. Seems like there are no obvious restrictions on the file you sent. I forgot to mention in previous posts that I have HD Suite 2018.
I gather from the responses here that the preferred method is just to import a different file type and trace over the lines. I do have PDFs of the plans. I realize that DXF files don't have 3D data and there would be a good deal of tracing anyway, but I figured with an import the dimensioning would be precise and leave less room for human error (something of which I have more than my share). If DXF has no real benefit then I don't see why it would be a "premium" feature.