Had a look at one yesterday, and I guess “technically” the Caltex forecourt sign is actually capitalised, but I think mapping convention takes precedence, as its not actually an abbreviation (like PTT). So, yes, make the edits.
JOSM auto-updates after a while. To force refresh look inside the cache folder and remove the file “*josm-thai-presets.zip” to force a fresh download on JOSM restart. It is located in the hidden folder user folder “AppData\Local\JOSM\cache”. Typically C:\Users<account name>.