I have a 2018r4 web app that I’d like to attempt to convert to Web 2.0. Is it important that I first load and save it with the latest version release v2019 before loading it with the latest release of v2021?
Nope
I opened a 2018 R.X in 2021 R2.1
As always YMMV 
If you really want to try to convert it by opening it in 2021R2.1 that’s up to you, but I would recommend copy and pasting things a few at a time into a fresh project first so that you can see what gets changed like Open to Opening and Action to Pressed or Mouse Down to Pressed. Sizing of text fields and Buttons and some other controls changes quite a lot. I found converting to be significantly more difficult than rebuilding.
EDIT: I kept the old app open in the old version and copied stuff to the new version fresh project.
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There is no real “Conversion”, Xojo attempts to make some adaptations, but if you have control arrays, or any other not supported featureas it will be a mess.
Start a new proyect and start from scratch, copy/paste only code.
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I converted my projects via Open rather than rebuilding and in hindsight preferred I hadn’t. There was so much moving around of every on screen element that it would have been faster to drag in a new item and change its name and events.
Rebuilding gives you a great opportunity to improve your project AS you’re going rather than spending the same time or more just to get where you are now.
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