No, this will not change the agent on already matched items. It will just fetch the same old metadata from the same agent.
To really change the match you need to either:
- perform ‘Fix Match’
- ‘Plex Dance’ the item(s)
- delete the library and re-create it
- ‘Merge’ the two posters of the same movie. Which poster is ‘selected’ first, will determine under which agent the merged poster will end up.