It's all based around open source projects virt-v2v and Migration Toolkit for Virt, and the typical target is OpenShift Virtualization.
There are various zero-copy options if you're using specific storage. In the best case the downtime for each guest can be as little as a few minutes. If the storage stars don't align then it can take a few hours per VM (but conversions happen in parallel, dozens or hundreds at a time).
[I don't have any specific knowledge about where this Tesco account is going. We have plenty of competitors. Everyone is dining at the Broadcom trough right now. Broadcom's "strategy" is absolutely baffling to me.]
Edit: Almost forgot that I gave a 5 minute lightning talk about it: https://pretalx.com/devconf-cz-2024/talk/SN93LG/