POP to IMAP process

You need to understand what it means to have an IMAP setup in a client.  You should google it for more complete discussions but basically all it means is that the email client always tries to stay in sync with the email server and vice versa.  Thus multiple devices all set up as IMAP in their mail clients for the same account all stay in sync with each other.  What happens on one happens on the other(s).  Read an email on one it get marked read on the device you read it on and all the other devices will see it as already read too.  Delete on one, it's deleted on all the others.

Generally to switch from POP to IMAP you need to create a new account in the client.  Thus at that point your client has both its POP and IMAP accounts set up.  Depending on how you set up your POP account you either have the stuff left on the server and also in your POP account or only in your POP account.  Either way the stuff is in you client's inbox plus any other mailbox folders you filtered (sorted) the incoming stuff into.

Once you got your IMAP account set up you can copy the POP account's mailboxes to your IMAP account within your client.  For the inbox copy the inbox contents, not the inbox to your IMAP account since the IMAP account already has its own inbox.  What happens as you do the copies the IMAP account will sync with the server thus creating those folders and inbox contents on the server.  Depending on the number of emails in those POP folders this could take a while since it's not only copying the stuff within your client it is also sending all that stuff to the server as well. 

Once the server and client are in sync there shouldn't be much lag in using your email.  Of course there's that first time step bringing additional email clients on other devices on line as well (why would you use IMAP for only single client access?).  In there case the copies will be in reverse, i.e., from the server to those other clients to get them in sync.  Assuming thos clients were also set up for POP you have to figure out how to get the differences in folder lay and contents in sync.  But so long as you remember what happens to one IMAP client happens to them all it shouldn't be much of a problem.

Note, the webmail always mirrors the servers.  So you can see this synch process happening if you view your webmail at the same time.  Create a mailbox folder in your IMAP client and you will see it appear in the webmail (there's a small lag in the timing though).  You can even create a mialbox in the webmail and it will appear in all the IMAP clients too.  It all works in any direction.

So there is no "archiving" to be done here.  What's on the server will end up in your IMAP setup and vice versa at the end of this process.  You don't have to delete the POP account in your client but you want to disable it from fetching new email.  You don't want it fetching the same email the IMAP account is also accessing.  It could get confusing.  It's you choice if you want to delete the POP account in the client after you switch over to IMAP.

Thanks!  You've confirmed what I was thinking and hoping would be possible.  I'll be giving this a try.