I currently have Lightroom 6. I need to use Lightroom Classic for an ad hoc project for the next 2 months. After 2 months, I will no longer need the updated version, and will want to revert to my previous Lightroom 6 version. Is this possible and if so, how is the best way to accomplish this?
The only way to get Lightroom Classic is to subscribe to the Photography Plan (20 GB).
This is a one year subscription, paid monthly. If you cancel early, you will be charged an early termination fee, which I believe is 50% of the remaining payments. So you will be paying $20 for the first two months, and a $50 early termination fee.
I'm not sure what happens when you install LrC on a computer that already has Lr 6 installed.
It might be deleted, and if it doesn't, it will probably turn into Lr CC 2015 (which was the subscription version of Lr 6) when you install LrC. This may get you into trouble when you cancel your subscription, Lr 6 (which has now turned into CC 2015) may stop working because you no longer have a subscription.
I think it's possible to revert it back to Lr 6, but it might require reinstalling, can't remember the details.
You haven't told us what OS you're using, but if you have a recent Mac, installing Lr 6 is not possible. (there seems to be workarounds in some cases, but not being a Mac user, I'm not familiar with them)
If this project involves using a camera that's not supported by Lr 6, I suggest that you use the free Adobe DNG converter, which will convert copies of the raw files to DNG files that can be read by Lr 6.
If you install LrC, it will upgrade a copy of your Lr 6 catalog.
This new, upgraded catalog cannot be opened by Lr 6, so you won't be able to see the work you've done in LrC when you end your subscription. But there are a couple of workarounds.
Export full resolution files, preferably Tiff or PSD, when you've finished the project in LrC.
You can also save metadata to files (select all the images and go to Metadata > Save metadata to files. You can then import the files in Lr 6. This will not save all metadata, but all your edits will be saved. But features you have used that don't exist in Lr 6 (like for instance Clarity, Texture. Select sky, Select subject) will not be visible in Lr 6.