I created regex replace patterns with some instructions on two websites. Feel free to give it a try if you wish. Make sure to have backups of your saves!
You will need a Save Editor of your choice for the full Raw JSON data from your saves on both platforms.
Additionally you need to know your Game Pass UID, Game Pass Username, Steam64ID, and Steam Username, to be used. If you then do it correctly, either through the links, or with Notepad++, you will end up with converted full JSON save data, to overwrite the save data on a save for the new platform.
Alternatively you can use regular expression / replace patterns for Notepad++ below.
Make sure to check the βregexβ tick-box and replace the following parts:
<GPUID>
<GPUSERNAME>
<STEAM64ID>
<STEAMUSERNAME>
Game Pass to Steam (NP++)
Find what:
(?<lid>"LID": *")[0-9]*(?<uid>",\s*"UID": *")<GPUID>(?<usn>",\s*"USN": *")<GPUSERNAME>(?<ptk>",\s*"PTK": *")[A-Z]{0,2}(?<end>")
Replace with:
$+{lid}<STEAM64ID>$+{uid}<STEAM64ID>$+{usn}<STEAMUSERNAME>$+{ptk}ST$+{end}
Steam to Game Pass (NP++)
Find what:
(?<lid>"LID": *")[0-9]*(?<uid>",\s*"UID": *")<STEAM64ID>(?<usn>",\s*"USN": *")<STEAMUSERNAME>(?<ptk>",\s*"PTK": *")[A-Z]{0,2}(?<end>")
Replace with:
$+{lid}0000000000000001$+{uid}<GPUID>$+{usn}<GPUSERNAME>$+{ptk}XB$+{end}
Any questions, please ask!