First off, we need more information like: what specific heat treatment is required... For example: Preheat, Post Weld Heat Treatment(PWHT)of any type or a change in any of the examples just listed... We still cannot specifically determine what you need to do without knowing what welding process is being used which is absolutely important in order to distinguish which table to look at in Sect IX in order to determine whether or not it's an essential, supplementary essential, and a non essential variable...
So if you can furnish the information on what type of heat treatment, and the welding process as well as if the welds are part a weld overlay build up to be used, we can then give you a definitive answer - that is unless you forgot to mention something which at times does happen in here and usually leads to a totally different resulting solution...
Now, if the WPS has already been written, and the welders have already been qualified by the WPS that doesn't list some form of Heat treatment and as you previously mentioned is required, then it's more than likely that you'll need to re-qualify unless it falls under a non essential variable and the only way to determine this would be to have all of the pertinent information available to then go into ASME Section IX and look @ the tables starting from QW 252 thru QW 265 and stopping at the table that refers to the process being used...
If you have access to ASME Section IX 2010, you can find those tables starting on page 20 all the way to page 46... More information please.
Respectfully,
Henry