Changing my POP accounts to IMAP - questions!

[TelephoneTree]

1. Do I need to do anything about my existing emails on my computer (mac) first? ie is there a chance they will all vanish?

1. How do I actually go about changing my accounts to IMAP??

Who do you have your email with? And why are you changing?

One yahoo and one with 123-reg forwarded via yahoo.

Changing because I want emails to delete on both computer and phone if deleted on one...

You should be able to set the settings on the device to say you want the email to be deleted (or not) from the server.
Gmail offers a choice (iirc) or pop or IMAP account, in the settings, but I'm not sure yahoo does that.