FWIW I asked a game master this question as well and got this response:
I see you are contacting us with a question about the upcoming Valor reset in WoW Shadowlands.
Valor and Conquest points are seasonal currencies and at the start of each new season will be converted and automatically added to your gold total. So most likely - as with the previous patch (Chains of Domination and Season 2 Dates and Information) - it will not happen immediately on patch 9.2 release date, but some time later at the start of the new season.
Hope that helps! ^^
The “most likely language” is a little vague but they go on to clarify it and also given that in season 1 valor got reset at the end of the first week of 9.1 (July 6), makes me think this season will be the same. Based on this I’m under the impression it will reset on the 1st of March.