If you’re going to suggest I update my libraries to use a new metadata agent and in the same breath state that existing items will not be affected then do not introduce any changes to existing items. Please sit down and take a minute to consider that this had to be said to you.
I change the poster for almost every new item in my libraries and have done so for years. Then along comes another dubious improvement and it’s all gone.
You should have also stated, in bold red text, that there is going to be a library scan. Some of us have parts of our libraries on remote systems where any sort of access equals an API call, and those are often limited. Scanning a whole library without any warning has the potential to make for a very unpleasant evening.
I just had my staff barge in all out of breath and inform me “Mr. Iratemonk, SIR! The Plex server is a goner”. SAD!
EDIT:
Just glanced at Should I "upgrade" my libraries? that had a screenshot of the prompt and noticed there was a checkbox for “Refresh all metadata” that appears to have been checked by default. So, I guess this one’s on me.