Adding the -ROAM switch to the command line "tells Pegasus Mail that it should ignore the configured drive letter when locating user mailboxes and mail files, and should instead use the drive letter of the drive from which WINPM-32.EXE was run" (quoted from the help file).  In other words, it allows a copy of a Pegasus Mail installation to be run as a portable installation.  The command line switch would look something like F:\PMAIL\Programs\winpm-32.exe -roam -a. 

All that said, if the USB device will stay connected with a fixed drive letter then the -ROAM switch is not necessary.

I am currently using Pegasus Mail Version 4.41 on a USB thumb drive on a Windows XP computer. As this machine can't last forever I will eventually have to move from it to my Windows 7 Pro replacement. Would my best route to this end be to upgrade Pegasus using the XP operating system in incremental stages ? I believe the software has been advanced about five times since I settled with 4.41.

To upgrade, do I just install the software to my external drive and then drag and drop in my "Mail" folder ??

That should work.  The installer will create a \MAIL directory which you can overwrite with the one from your thumb drive.  Don't worry about doing any configuration as the configuration files will come over with the \MAIL directory.  The requirement is that you use the -ROAM command line switch which I assume you are already using.  The installer should add that switch to the shortcut automatically.

I suggest installing v4.63. Version v4.70 has a few issues that are being worked on.  Hold out for v4.71 or until you need the new SSL implementation included in v4.70. 

Thank you very much for this advice. I only just found the response as I must have neglected to set this thread to send me alerts.

I am not sure if I have enabled the ROAM  command on my installation. I'll have to find out how to do that again - I believe you can only easily set it using an XP machine and I have a pickle on that that I need to sort out - starting at these forums with a query about what it is safe to delete.