RE: Migrating Custom Fields from CRM 2013 to Dynamics 365
Hi Huzefa,
I am wondering what you are trying to do.. those fields are, probably, added to the forms/views/workflows/etc. You might just want to create a solution, add opportunity entity to the solution, export the solution, and import it to the 365 version. Those solutions should be compatible:
msdn.microsoft.com/.../gg334576.aspx
If, on the other hand, you just wanted to move the fields (not the forms/views/etc), you might still do exactly the same, but, before doing that, you might create a backup solution in the 365 environment (add opportunity with all assets there) and export it as a backup. Once you've imported 2013 solution to 365, you might re-import that backup solution to 365, too. That should restore all the forms/views to the 365 version, but all the fields brought there from 2013 would still be there.
PS. That said, I'd test all this in the sandbox