Your comment led me to run a search and found this for anyone else interested:

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Hi everyone! I was really dissapointed when i found out that you won’t be able to transfer your saves between MS Store and Steam versions, even though you have to use your XBOX account for both of them anyway. So i found a way to do it myself! Updated to add a way to find the save file after you get a lot of cars.

I presume you already have some progress made on the MS Store version.

Launch the steam version at least once, up to main menu, then exit.

Locate the folder where steam save files reside, it should be \userdata\\1551360\remote\\, make a backup of those 3 files and remove them from the save folder.

Locate the folder where MS Store saves are, it should be %LOCALAPPDATA%\Packages\Microsoft.624F8B84B80_8wekyb3d8bbwe\SystemAppData\wgs\\\

Turns out Microsoft has decided to store every car you own in a separate folder. This makes finding the actual save folder a bit more difficult. The way you can recognise it is either look for a file that is around 900kb in size, or has a container.208 file.

There will be 4 files in the MS Store version folder, we only care about 3 of them - those with long file names and no extension.

Copy them over to the Steam version folder.

Now we look to the filesize. The biggest of the copied-over files is our User_XXXXXXXXXX.ProfileData, change its name to the name of the file you copied.

Next ones are a bit tricky. You have to open them, and User_XXXXXXXXXX.TransactionLogFile and User_XXXXXXXXXX.VersionFlags you copied earlier, in a text editor like Notepad++, then compare their content. Change the names accordingly to User_XXXXXXXXXX.TransactionLogFile and User_XXXXXXXXXX.VersionFlags, it should be obivous which one is which after you open them.

Launch the game, you'll be able to continue playing using the Steam verison from now on!

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Wish I had found this earlier! 🤦‍♂️