Thanks, I figured that might be the case.
If it turns out that swapping buffers is trivial that will be great ... otherwise I'll work around it.
One workaround is to use SVN (Tortoise in my case) CONFIG which allows "external DIFF program" with command line.
I have that set to
C:\Program Files\SlickEdit Pro 23.0.0\win\vsdiff.exe
which then gives me old-on-left, and new-on-right by default, but it has parameters for Old / New filenames (%base and %mine) so possible to pass those to DIFF as appropriate.
In the past I've always used Window Explorer and Tortoise ... but the new version has a significant "pause on launch" (its about 2 second each time) which is why I tried SVN-from-SlickEdit (which does the DIFF on each file "instantly"). I'm guessing, but seems that its the "launch" time rather than the Compare time ... might be related to having tons of stuff open on my PC.
Happy to try any suggestions that might could improve the launch performance - then I can just revert to Windows Explorer / Tortoise route.
I've tried having both VSE and VSDIFF open already (comparing something else), in the hope that "in memory" would solve it, but doesn't seem to make any difference.