Will Email Export Overwrite

apologies upfront - not a simple scenario.

background : using the same coax for internet for 25 years and email address for 17 years.  I have emails going back to 2008 when Comcast bought previous operator/ISP.  My mailbox hovers around 90% of the 10GB limit. I regularly have to go in and remove old files/attachments to avoid missing/dropping new messages. Unlike Gmail,Outlook,Apple,Proton, etc ....Comcast will not accept monthly subscription money for increased storage.

In 2022, I did some sort of copy from Xfinity email to Apple/icloud email.  For the most part.....it seemed to work.  Over the past 3 years when I get a notification from Xfinity warning me about mailbox size, I look for folders that are not used much, I check contents are all available on Apple/icloud, then I delete the folder(s) on Xfinity to free up space there.

Now I am preparing to finally move. I have Gmail and Yahoo accounts but looks like I will stay with Apple/icloud.  So, I have 3 questions.

1 - if I use the Xfinity Export Portal and follow the instructions for "export data from Comcast.net to Apple Email" .....will this 2nd export clobber anything I copied before ?

     as example "Sent" folder.  If Sent folder already exists at the destination, can the copy just use that folder ?    ...not delete and create new Sent folder.  Think this goes to how IMAP and the ShuttleCloud tool works.

2 - When that is complete, I will set Auto-Forward to the new address (gulp).  I will probably check the "keep copy of forwarded messages" for few days in case I need to backout if things are not working.

      I will still have my Xfinity service to the home.   So, my account and email should all remain intact - is that correct ?

3 - Let's say I decide to move in the future and at that point I don't have Xfinity service in the new location.

      My understanding is that my email account, email address and email forwarding info will remain as long as I login every so often.  Is that correct ?   Must login must be once a month, once a year, ... ?