Can't cancel Ultimate Game Pass and cannot get into old account

If you still have access to the email account related to your MS account you can reset the password and create a new one.

Ultimate Game Pass can't be converted back to Live Gold and if you cancel Ultimate Game Pass you will lose the Live Gold, so it is best to turn off recurring billing and wait for the Game Pass subscription to expire then get Gold after that.

When you purchased the promotional price, anything you had on your account was converted to Ultimate Game Pass (that is stated in the promotion) and cannot be converted back. So by confirming your purchase for the promotion, you agreed to that. Cancelling
the subscription would cancel your Ultimate Game Pass which included the now converted gold. You would have to purchase a new subscription.

Since Ultimate includes gold, game pass for consoles, and game pass for pc, anything you already have on your account is cancelled and converted to ultimate when you upgrade. By completing the purchase for ultimate, you agreed to that and cancelling ultimate
would cancel anything that was converted. You will have to purchase game pass for PC again if you want it again.

If you buy game pass Ultimate, it will convert your existing subscriptions to the Ultimate sub. Once it is all done, if you check your subscriptions at accounts.microsoft.com it will show the new Ultimate pass, and the old Subscriptions as Cancelled.