Resolution:
If you are migrating from and Old SAP version to a newer SAP version, the folder structure under which the schema is getting generated will be different and you need to download the 620 schema to the same old schema folder (45B), download it using Custom Folder (Text field under Adapter Services folder). In Custom Folder, give the folder name as “45B” and try downloading the schema. The above option is provided in R/3 Adapter 5.1.1 HF 3 and higher versions

                                                      Or

2. Another work around is to configure a BW process which would act as a bridge between the 2 projects. Created a BW process which will subscribe the RV message with old schema and map the same to new schema. Convert the project into ‘*.dat’ and use Convert ‘*.dat’ to Files utility of the Designer.