[chmurf]

you can upgrade your AP, either in person, or over the phone using he passholder line (US number)

if you do it over the phone, you'll be able to use your pass to enter the parks but you'll need to go to guest services to exchange your pass, since it will be a different color.

Now, for the (maybe) bad news. Uprades to higher tiers can only be done at the renewal period.
this does not mean that you won't be able to upgrade in april, but it means that your Premier AP will have the same expiration date. So your Premier AP will still expire in June this year

and why is it possibly a bad news ? Because you will pay the full upgrade price, and not a price calculated on the number of days remaining on your AP

the price you'll pay, will be that of the difference between Premier and Preferred as it was at the date you purchased your Preferred (or upgraded it)
so you'll pay the full upgrade price despite having just a few months remaining