Thank you very much for your input.
I will probably leave both the earlier version of jQuery (for older pages) and use the 3.4.1 for new pages.
I am a little perplexed as to what to use instead of the jQuery UI for the spinner (I know you don't like them).
I will to search online for another way then.

>>I would like to update my project with the latest version of the jQuery and jQuery UI code. On the page, Download jQuery, the version is 3.4.1 And yet, on the sample pages of the jQuery site, the samples use version 1.12..
>>Should I still replace all references of 1.12 with 3.4.?
>
>
>jQuery 3.4.1 is the latest version. There are significant changes in 3.x over 1.x that likely will break existing functionality. If you just use the basics you might be fine (selectors, ajax, basic assignments), but if you heavily work with event handling (on,live, etc) and promises you'll likely see things break.
>
>I'm not sure what the state of jQuery UI is - but it looks to me that it only works with jQuery 1.x. jQuery UI looks effectively abandoned. Last updates I see are 3 years ago with some comments about a v 1.13 with minimal new features that never were done. It's done. Pretty sure that using jQuery 3.x with jQuery UI 1.x will break stuff since jQuery UI uses a ton of jQuery's functionality.
>
>+++ Rick ---