1) UR operates in an identical way regardless of the file extension of the files you open in it
(they must be SQLite format and contain UR database schema, but the file extension is irrelevant)
2) The 64-bit edition of UR has existed since v6.0 (Sept. 2019)
https://www.kinook.com/Forum/showthread.php?t=5602
You can download the 32-bit edition from
https://kinook.com/Download/UltraRecallProEval.exe
and the 64-bit edition from
https://kinook.com/Download/UltraRecallProX64Eval.exe
3) You can always view the detailed history of changes here:
https://www.kinook.com/Forum/forumdisplay.php?f=27
which is linked to at the bottom of
https://www.kinook.com/UltraRecall/Manual/?version6.htm
Note: See the
history page on the web site for full details.
4) UR / SQLite can handle much more than 50,000 items. See
https://www.kinook.com/Forum/showthread.php?t=709
https://www.kinook.com/Forum/showthread.php?t=3627
https://www.kinook.com/Forum/showthread.php?t=3890
5) Nothing has changed with UR (the executable is from Aug. 2021 and did not change with the latest update --
see
https://www.kinook.com/Forum/showthread.php?t=5776),
so if you are unable open the files in another SQLite front-end, something must have changed on your end.
6) I used a binary editor to change the signature of the Intro sample DB to SQLite Format 3,
renamed it to Intro.db (in attached ZIP file), and was able to open and query it in
SQLite's sqlite3.exe, Navicat, DB Browser for SQLite, and SQLite Studio and UR
(downloaded and installed latest versions of each today).