Susie --
Much of the legacy code that I upgrade also has many Fortran extensions offered by some vendors ... they are NOT part of the formal Fortan language and their usage makes your program non-Code compliant ... so, you encounter problems trying to convert to a different vendor's compiler. Examples that I encounter include Q-formatting, Encode/Decode, Dispose and <>. To avoid this type problem, you must convert those statements to the formal Fortran language instead of using the vendor's extensions.
Instead of OPEN(... DISPOSE=DELETE ...), try CLOSE(... STATUS=DELETE ...). That might solve that problem.
Good luck -- Mark