Hey there @DangerDays11
There is no option to upgrade an existing purchase so you would have to cancel and then pre-order to change to Ultimate Edition.
You might not lose any pre-order items because you are not re-ordering the same pre-order edition.
To be certain you should go to the Xbox store online and inquire with them as I have read on EA's website you do risk losing the rewards if you cancel and re-order but that might be only if you cancel Ultimate Edition then re-order Ultimate Edition (with EA Play or for some other reason)..
If you do pre-order Ultimate Edition you will get the full 7 days early access starting from 22nd September.