Hello Jack. Thank you for your response. I spoke via Chat with an Apple support person since I have Apple care and stated my question about how to transfer my existing backups from my external hard drive to a new 2TB Seagate external hard drive and after checking with other support agents he said "Apple does not support what you are asking. He recommended that I plug my new hard drive which is formatted in exFat and click on time Machine and it will reformatted APFS which is the only format recognizes by Time Machine." So I decided to proceed with that.
If for whatever reason I need to look for any old backups (files) that are in the existing backup drive to be replaced I will just switch them.
As far as WD being a better external HD than than Seagate that may be true, however I have had bad experiences with WD products.
My son gave me a 4TB WD portable drive a couple years ago and was using it as NTFS format drive with at least 3 Gig of files, family photos videos from overseas trips etc. One day I was downloading some of my files to my Microsoft HP PC something went wrong where it got stuck in one of the downloaded files. I tried everything so it can continue and nothing I did would continue. It was frozen spinning on that file. Since I could not eject the WD hard drive I shutdown my old HP PC disconnect the Hard drive then boot it back on my PC. When I tried to reconnect back the WD external drive all the sectors heads had bad reads. I took my HD to a place that specialize in recovering data from a damage drive and they could only recover only two sectors not the ones I had my photos and videos.
This year he gave me a brand new WD Easystore 4TB. I have backup some file and movies and series he gave me. When I tried to watch it from the WD drive is so slow executing the video for you to see it like in slow motion. The same goes to music videos or MP3 file. I have to download the files what I want to see or play to my Apple iMac 27" computer and see it from my PC or my TV screen via Air Play fast as normal. So I got not to trust WD products. I never had any of those issues with Seagate products.
Thank you anyway for your advice.