I edited my post above because I was wrong. I thought that you couldn't add 'no expiration' to tix once bought and, having read a bit more, it seems that you probably can, although this is about 'magic your way' tix:
"In general, guests can upgrade to any type of ticket, provided the new ticket is of equal or greater value than the old ticket by simply paying the difference. However, no refunds will be issued if the new ticket is of lesser value. So, yes, guests who purchased Walt Disney World tickets before the new Magic Your Way tickets became available will have the option to upgrade to a new pass."
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http://www.wdisneyw.co.uk/mywfaq.html
It might be a case of simply making it clear to them by describing what you want in a different way. Instead of saying you want to use your Ultimate tix next year, or that you want to swap it for Ultimate tix next year - say you want to *upgrade* them to a more expensive (or equal value) ticket, which does not expire this year.
Of course it may be that the Ultimate tix have totally different terms&conditions because they are for UK people only. If you think about it, since the Ultimate tix are not available to people in the US and are often better value than the US option, they must be governed by different rules than the standard tickets. One of the rules being who is allowed to buy them...