Yes. I've done it and gone back 'n' forth a few times. No reauthorization necessary going back to 1.5.1.c.
Here's my method.
I have 2 different "Sage" directories backed up to an other location.
In my case I have a folder called "Stylus_SAFE" on an other drive and inside of that I have folders called 1.5 and 1.7. Inside of each of those is the sage directory relative to that particular version.
If I need to be 1.5 then I delete my (1.7 ... same as 1.8) sage directory from the "real" location ... i.e. the location that Stylus expects it to live, and replace it with the 1.5 that is backed up in my Stylus_SAFE folder.
As for the .dll it's as simple as having them both in the same location but renaming the one that you're not using to .dll_NOT at the end.
I then rescan my plugins, just in case ... and all is well.
If you haven't made a safe copy of your 1.5 sage directory then I suspect you'll have to somehow rebuild it,
(not sure if the new data will work with the old plugin so I opted to be safe rather than sorry) or reinstall it ...but be aware that your multis and any other patches you've saved need to be backed up if you want to keep them.