Salvage data from a reverse connected thermocouple?
There are 2 problems

1. Negative supply rail

... or possible lack thereof (as @Ignacio had mentioned above).  AD595 sees the reversed thermocouple as reversed \$\Delta T_{TC}\$.  If AD595 has a negative power supply, it can deal with negative temperature.  From the datasheet, it's not immediately clear for me how AD595 deals with negative \$\Delta T_{TC}\$.

By the way, negative \$\Delta T_{TC}\$ can occur at positive temperatures, when AD595 is at a higher temperature than working end of the thermocouple (e.g. 40°C for AD595 and 25°C for working end).

2. Cold junction temperature

As you know, AD595 measures its own temperature and does cold junction compensation (CJC) internally.  The output is the difference of 2 measured values, and you don't know what they are individually.  If the thermocouple is connected correctly, the output is

\$V_{out}=A(\Delta T_{TC}+T_{CJC})\$

where
\$V_{out}\$ is the output voltage
\$\Delta T_{TC}\$ is the temperature difference between the working junction and the cold junction
\$A\$ is 10 mV/°C

If the thermocouple is reversed, the output is

\$V_{out}*=A(-\Delta T_{TC}+T_{CJC})\$

Correction \$V_{out, corrected} = -V_{out}*\$ would have an error of \$2AT_{CJC}\$.  My gut feeling is that that wouldn't be good enough.

If by some luck you have recorded temperature from some other sensor near the AD595, you would have an estimated value \$T_{CJC}*\$.  Then you can try

\$V_{out, corrected} = -(V_{out}* - 2AT_{CJC}*)\$

Finally

Plot your voltage and post it here.  May be, we can spot a better answer.