Hi Stephen,
Thanks for contacting WPML support, I will be glad to help you.
It is not recommended to use multiple multilingual plugins at the same time as it can cause compatibility issues or even fatal errors.
I'm afraid that we don't have any docs about how to migrate from Weglot to WPML, so it is better to activate only one of them on the site.
However, you can test it on a staging environment and check if they can work together. Please note that we didn't test this and we can't guarantee that it will work without any issues.