I've been experimenting with this today and I'm not sure I completely understand the results. My primary Rogers e-mail is only used for advertising and web site sign-ins so is set up as POP3 with filters to forward interesting messages to other IDs. This is done through Thunderbird. I have a test Rogers e-mail ID that I use in order to leave my main e-mail undisturbed.
I set up the test ID under Thunderbird as IMAP. This allowed me to select OAuth2 as the authentication method. When I did this, it reported that OAuth2 was not supported for this account. OAuth2 is not offered for the POP3 account.
I then set up the test ID on MAC OS Mail (El Capitan), again using IMAP. OAuth2 was not offered as an authentication option. I read in the Thunderbird support that it supports OAuth2 for POP3 accounts, but does not offer it as a configuration option if the e-mail server does not support it.
I conclude from this that Rogers/Yahoo have not yet enabled OAuth2 and won't do it until the deadline date. At that point, OAuth2 would be enabled. But how does the authentication protocol get changed on the client? I read the Wikipedia article, and then read a basic introduction on the Oauth.org site. It appears that when OAuth2 is used the first time, the customer will be asked to authorize access to the e-mail resources by the application. I can't figure out whether OAuth2 has to be enabled on the client or whether it will be automatically enabled by the protocol.
I guess I just have to wait until the deadline date to see if it all works.
My contingency plan is to set up a new GMail account and change the Rogers/Yahoo e-mail to forward all mail to the Gmail account. This has a risk to it as Yahoo has on occasion disabled mail forwarding.