If you have some controls on a page, these controls are placed inside placeholders. When the template is changed the system is trying to find placeholders in the new template that have the same id as the placeholders from the page. If it is not able to find such placeholders all of the controls are placed in one free placeholder on outside any placeholders if it could not find a free one. This is the default behavior of Sitefinity.
We could add a feature request for you regarding the ability to place the controls on a default placeholder when the template of the page is changed.
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system and vote to affect the priority of the items