If you're sure the HNY is only £17 to upgrade (ie you have booked the SL under the same offer that the HNY is being priced at) then definitely, I'd upgrade .
If you didn't book the SL under the same offer as the HNY price (ie 40% off deal etc) then you'll usually have to cancel your SL booking, lose 10% of your total for the SL as a cancellation fee, then make a new booking for the HNY.
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