Let's clarify this once and for all.
First rule of Email Club is:
If you're going to use more than 1 access point for your emails, then you need to understand that POP is NOT going to serve your needs and you have to adopt IMAP.
Second rule of Email Club is:
If you use POP and have a habit of not bothering to read your messages, a firmware upgrade or new device or new PC simply means it WILL re download ALL your messages including unread ones.
Third rule of Email Club is:
If you don't know what IMAP is, Google it.
It's fascinating how people seem to keep sounding like a broken record when it comes to this issue.