What if it included the date of most recent update? Worst case, you could store the user's most recent TOS agreement version or date identifier, and force them to agree after logging in if they want to continue. This seems to be how Apple operates for the App Store.
When I was at Yahoo years ago, and we handled billing for premium servies, we'd explicitly store the version of the TOS the user had indicated express consent to as an extra precaution (express consent in this case meant the user had ticket a checkbox to confirm they agreed and then submitted the form).
First of all, the login experience should not change if the TOS is the same as before.
If the TOS has changed, you could accept-reject the login (lock the credentials in the fields) and say "to continue, please read the TOS change and press 'accept' to continue". Or just have this as a separate screen after the login.
