There is no need to change email addresses if you change your ISP.
I still use the e-mail address I had when I had a dial-up internet account in the early 90s. Although I went to BT for ISDN and later ADSL service, I still pay a small charge to ClaraNet for use of their SMTP servers.
Also, I have 2 domains registered to me for which the hosting company provides a forwarding service for emails addressed to me at those domains to my ClaraNet address. If I decide to terminate the Claranet account, then I can easily change the forwarding details with the domain hosting company to forward to the new ISP provided email address. One of my domain email addresses is the one I give when I register to on-line services and the subsequent spam sent to that address does not bother me since it does not get forwarded to my real everyday account.