Whether you are changing ISPs or just going to a free email system, the two major problems people see are 1. How to get their address book from the old provider to their new email provider and 2. How to tell everyone you have changed email addresses.
To move your email address book from one provider to another just involves creating a file on your PC. Go to your contacts at your current email provider and there will be an option to export your contacts. The common way that is generally accepted by different email systems is to create a .CSV file. Make sure you remember what you name it and where you save it. This also works good as a way to backup your contact list. Then go to your new email provider and go to contacts and look for an import option. Choose that and it will ask for your .csv file and browse to it and choose to open or import and all your addresses are now in your new system.
Although my main email address of dwight-watt@att.net has been mine since 1996, I recently used the above steps to put my address book in Gmail also as I have an account there for many years I use as a secondary email address. Now my contacts are in both places.
To let your contacts know that your email address is changing, I would suggest using the old email address to send the notice as some have security to only allow email from recognized addresses. Create an email and then using the TO or BCC button click at the top of your list and scroll to bottom and with shift key depressed left click. Then choose to put them in TO or BCC. BCC will be better so you don’t send your address book to everyone and also make simple for spammers to get. Then write message with the new email address and send. On a few systems you may have to send only a part of your list on each message as they will block as going to too many addresses.