I am changing ISPs and will be dropping two email addresses I use as AppleID's. Can I keep them as AppleID's and use a rescue e-mail address from a new ISP? What are the downsides if I have an iCloud address as an alias under one of those addresses?

I am about to change internet service providers and will be dropping two e-mail addresses that I use as Apple ID’s. One is used solely for iTunes and the other is used for iCloud, mail, calendars, etc. The second has two iCloud email address as aliases. I will be going to a new ISP and will establish an e-mail address from that ISP as a backup in case the iCloud addresses are not available.

It has been suggested to me that I should continue to use the e-mail addresses from the old ISP as my Apple ID’s. And that I use the e-mail address from the new ISP as a rescue e-mail address for those Apple ID’s to get any communications relating to the ID’s. That seems like a simple and straight-forward way of handling the change and would minimize the chance of complications from changing the Apple ID’s. But, are there any downsides to handling the change in this way? If so, what are they?