It's not so easy. First of all, there is a 250€ change fee:
CHANGES ANY TIME CHARGE EUR 250.00 FOR REISSUE.
And then, even if you can use the historical prices, the original rules must be met:
REPRICE USING FARES IN EFFECT WHEN TKT WAS ISSUED
PROVIDED ALL OF THE FOLLOWING CONDITIONS ARE MET-
1. NO CHANGE TO 1ST FLIGHT COUPON
2. CHANGE IS BEFORE ORIGINAL SCHEDULED FLIGHT
3. A3/AC/AI/AV/BR/CA/CM/EN/ET/EW/HO/IN/JP/LH/LO/LX/
MS/NH/NZ/OS/OU/OZ/SA/SK/SN/SQ/TG/TK/TP/UA/ZH ANY FARE
TYPE EXCEPT EOU/ERU ARE USED
4. PUBLIC FARES ARE USED IF TICKETED FARE IS IN
PUBLIC TARIFF. PRIVATE FARES ARE USED IF TICKETED
FARE IS IN PRIVATE TARIFF
5. NEW TKT HAS EQUAL OR HIGHER VALUE THAN PREVIOUS
TKT
6. ALL RULE AND BOOKING CODE PROVISIONS ARE MET
7. ADV RES IS MEASURED FROM ORIGINAL TKT DATE TO
DEPARTURE OF PRICING UNIT
There are a dozen of these rules and they're not easy to read. I don't think you'll be able to change to a flight that you can't just book outright right now.