Logging Xbox 360 profile into Xbox One

I've been doing some reading, and it appears as if you can be logged into one Xbox One and one XBox 360 at the same time. You cannot be logged into two Xbox Ones or two Xbox 360s at the same time though. So you may be fine using the same profile on a XB1
and XB360 at the same time.

You can be logged onto one profile on one Xbox 360 and one Xbox One at any one time. You cannot be signed into two Xbox 360s at once or two Xbox Ones at the same time, but one of each is perfectly fine.

You will be able to go back. In fact you can have your profile logged into Xbox Live on one xbox 360 and one xbox one at the same time.

It sounds like you're attempting to create a new account instead of logging in with the existing one. On the Xbox 360, you want to use the option that says "Download profile". From there, you log in with the email and password, and you should be
good to go.

Here are the official steps for adding/removing profiles on the Xbox 360: https://support.xbox.com/en-US/my-account/manage-gamertag-and-profile/add-remove-profile-xbox-360

I am assuming this is what you mean.

Back in the 360 days you were not required to have an online MS account connected to your xbox profile (it is now required with the ones).

The 360s used to have the capability of have local only profiles that were attached to the console themselves and could not get online and play multiplayer.

Again I am assuming this is what you are talking about.

The following link will bring up a support article with instructions on how to turn an offline profile into an xbox live online profile.

This does have to be done on a Xbox 360.

Offline 360 profile to online profile