Here's your code, in the top two lines, exactly as I changed them (simply swapping the two red "jen" and "rekt") words in the default rwgmixer.xml file in the main game folder.
You could also just switch the "forest" biomeTag with the "burntforest" tag instead, if you wish; it will give you the same result.
<prefab_spawn_adjust partial_name="trader_jen" biomeTags="forest" bias="20" min_count="2" max_count="4"/> (was "trader_rekt")
<prefab_spawn_adjust partial_name="trader_rekt" biomeTags="burntforest" bias="20" min_count="2" max_count="4"/> (was "trader_jen")
<prefab_spawn_adjust partial_name="trader_bob" biomeTags="desert" bias="20" min_count="2" max_count="4"/>
<prefab_spawn_adjust partial_name="trader_hugh" biomeTags="snow" bias="20" min_count="2" max_count="4"/>
<prefab_spawn_adjust partial_name="trader_joel" biomeTags="wasteland" bias="20" min_count="2" max_count="4"/>
<prefab_spawn_adjust partial_name="sawmill_01" biomeTags="forest,snow,burntforest" />
<prefab_spawn_adjust partial_name="rural_outdoor_wedding_01" biomeTags="forest,burntforest,snow" />
It was that simple BUT it means starting a new map for it to take effect. I would also suggest doing it as a mod in the Mods folder, to prevent it getting wiped in any upcoming update.
pj