I dont understand why a program would change the file format of a file nor why manually changing the extension didn't work.
Anyway, try the file to open in Photoshop by going to the file menu and choose Open As... Then navigate to such a file and choose PSD as the file format.
Hope this works.
Adding to what Ronald said, you should be able to install CC 2018 alongside CC 2017. Plenty of users--especially those who do mission-critical work---do this without problems.
It sounds like the root of this is broken file associations. That can happen if you uninstall older Photoshop versions with a new one still installed. The older version takes file associations with it, leaving the new one orphaned.
Uninstall all the Photoshop versions you still have, in reverse version order (newest first). Then reinstall the one you want.
In general, Photoshop should always be installed and uninstalled in the appropriate version order. If you don't it usually works out - but not always. I've had this happen to me once, and since then I always left the old versions installed. If they pile up after a while, I clean everything out and reinstall the latest.
I THINK I recall deleting CC 2018 either FIRST with CC 2017 afterwards,
OR I deleted CC 2018 with NO CC 2017 installed (at that time) on my
computer. But I've had so many problems between
installing/deleting/trying to get 2018 to work, 2017 to work again (as
it did before 2018 was installed) and so on... my mind is fried. Thanks,
Nevertheless...I am TRYING to find a workaround that does not involve
installing CC 2018 at all, preferably. Because at this point I just feel
it's going to mess up CC 2017 which I currently have installed and which
continues to work for me satisfactorily.
Best result: CC 2018 DOES install ok AND it doesn't mess up 2017... I
then open those 'orphaned' files in 2018, change them to some file
format such as .TIF (?), then try opening them up in CC 2017, saving
them in THAT version's .PSD. If all that works, I would like to NOT have
CC 2018 still installed, and delete it... while keeping CC 2017. Which
leaves me with files with .PSD extensions that I can open with CC 2017
(and, I presume -- but I can workaround if not -- OnOne's program). And
open with -- later if it ever gets updated/corrected sufficiently!! --
CC 2018, should I trust that I can then download (again), too.
Does that 'sound like a plan,' fellow users?
And D Fosse, when you advise
>In general, Photoshop should always be installed and uninstalled in
the appropriate version order.>, do you mean install in this order: X,
X1... but delete in THIS order: X1, X?
Opening files created with Ps CC 2018 WITHOUT 2018
created by D Fosse <https://forums.adobe.com/people/D+Fosse> in
/Photoshop/ - View the full discussion
Install 2015 > 2017 > 2018
Uninstall 2018 > 2017 > 2015.
Of course, when installing you can go directly to the one you want without any prior versions. But anything you do install/uninstall should follow this sequence.