Restore Backup of 90D to 200D

officially no fortinet doesn't support that.

But once you understand that the config files is just commands which the fortigate device reads, then you can get little creative and make this work.

Open your 90d config file and make all the changes in the config as per your requirement on the 200d for eg: interface ips, switch mode/ interface mode, lan and wan ports, policy source and destinations etc. Once it all matches you can upload the file to 90d.

It'd make things easier if you make sure both the devices are on the same fortios versions as well.