Okay. Mission Accomplished!
There were a few interesting aspects of the exercise I thought I'd share here.

First, Paragon Partition Manager was no help in converting the disk. Not due to any flaw of its own, but my reinstall options, from the hidden partition and via the recovery disks, both automatically selected FAT32 format. If either of them offered me the option of NTFS I must have blinked and missed it. So I used convert.exe in the end after all. Well, it worked.

The program at the link jephree posted above is worth having, although in my case I think the tech who fixed my earlier problems must have realigned the sectors. It was done in just a few minutes.

Here's a weird one: all throughout the conversion process I was able to use my wireless Labtec keyboard and mouse. Normally I have to dig out the original Acer USB parts to do anything. Since I'd wiped off all the programming and drivers, this has me seriously confused!

Paragon couldn't help me with the conversion, but it did reallocate disk space without any hassles. The C: disk now has an extra 100GB, and the D: an extra 50. No problems.

The hidden partition still shows quite a chunk of storage space as used, but when I tried to restore to an earlier setting (I was hoping to get back a couple of lost items) it did nothing. It could restore to original settings but that's all. Any ideas how I might get access to the restore, if it still exists?

There have been a few annoying changes in the display, and one by one I'm sorting them out. But the closing screen has changed from the three buttons format to the dropdown menu format. Also, I'm getting the sign on with password screen at opening. I'm not sure what I did to bring them on.