I would assume that it will supersede the model that they currently have. So eventually 2.5 flash will be the new and improved 2.5 Flash rather than 2.6.

Same way that openai updated their 4-o models and the like, which didn't turn out so well when it started glazing everyone and they had to revert it (maybe that was just chat and not api)

Even if it was just chat and or API I have used the API and I know that they have at minimum added the retraining date and time that they could just affix to the Gemini 2.5 Flash and Flash-Lite because when I use the API I have to verify that the upgrade of the backend system didn't break anything and pinning versions I assume is pretty common.