The typical rule actually says "first fare component" - see LondonAndy's post. AIUI, if your first fare component covers all the sectors of AMS-LHR-JFK-LAX and you change only the JFK-LAX flight, that would be a change to the first fare component and trigger a compulsory re-price at current fares.

If you don't change the first fare component but only make changes to subsequent fare components, then the typical rule is more generous than "original fares". Again, you can see from LondonAndy's post that the ticket is either re-priced using original fares or current fares, whichever is lower.

"Original fare" doesn't necessarily mean that you'll get exactly the same fare. If you booked the inbound half in I class but you want to change to a date that has no I class but has R class, you can be re-booked into R class with the ticket re-priced using the R class fares in force on the original ticketing date, if lower than current R class fares.

Also, a ticket re-issue will mean that the TFC are re-calculated at the date of re-issue, so whatever fares are used there can often be small differences in the TFC payable.

All of the above applies if the changes take place before departure of the first flight. After that, there's a different set of rules.