How to change keyboard layout using visual online editors VIA and REMAP
VIA and REMAP are great online visual tools to re-define QMK-based keyboards. All current ThumbsUp! keyboard support both tools by default.
Configuration File
Download JSON file for your board from here: https://www.dropbox.com/scl/fo/2s8vz5tq5lffmpjuqbz2c/h?rlkey=71amow1mhhwfe54c0ywx59bfc&dl=0
v2 Atmega: rev2_atmega_via.json
v2 RP2040: rev2_rp2040_via.json
v3, v4: rev3_rp2040_via.json
v5, v8: rev5_promicro_unibody_5x12_via.json
v6: rev6_rp2040_5x6_via.json
v7: rev7_rp2040_5x6_via.json
How to use VIA
In web-browser navigate to https://usevia.app/:

Click on Settings icon:

Click on "Show Design Tab" toggle, so it is moved to the right:

"Design" icon should be visible on top of the screen:

Click on it, this screen should appear:

Move "Use v2 definition" toggle to OFF (left) position.
Click on the "Load" button and select the JSON file downloaded before.
A pop-up window may appear, select your keyboard and click Connect:

Click on Configure icon:

Modify your keys, they changes should become effective immediately:

How to use REMAP
In your browser navigate to https://remap-keys.app/configure.

Let the site to modify your keyboard, click on "+KEYBOARD" and connect your board:

If the site asks for the keyboard definition file, click on "IMPORT (.JSON)" and specify the configuration file downloaded earlier:

Modify the layout:

Once the layout is ready - flash it by clicking on "Flash" button in the top right corner:

Please note these steps are a temporary solution. Once I register my boards with VIA and REMAP there will be no need to upload JSON file.