Attempting to move an accepted PhD to another university
Universities handle their PhD admissions independently. That is, if you don't want to continue your PhD at the university where you are currently admitted, you have to find a supervisor at the other university and get newly admitted there. If you already have research results from your current place, it may be possible to integrate them into the new PhD project, but that should be discussed with both the former and new supervisor. Such a move would not really be "frowned upon", as it can happen because of various understandable reasons, but in case you had already started your PhD, some people may interpret this as failing your first project and may have questions on this later during your career.

If you were in contact with someone about starting a PhD at their place, but they don't follow up, it may indeed mean that they lost interest, or maybe don't see a chance to have you doing this, for example because of a lack of funding or supervision capacity. Best would be to explicitly ask whether they still see a chance of having you as a PhD student.