CA IDMS has used the EXCP access method instead of BDAM for a number of releases now to access IDMS non-VSAM database files.
The files are defined with a dataset organization (DSORG) of physical sequential (PS).
The structure of the files has not changed with the use of EXCP, it is an access method, like BDAM and VSAM.
In older releases, IDMS VSAM was used because it supported extended addressing and BDAM didn't but now we support it with EXCP.
EXCP is more efficient than VSAM. The majority of customers are using non-VSAM database files.
Note that IDMS VSAM files are different to native VSAM files. Existing VSAM files accessed by CA IDMS are referred to as native VSAM files. They are not formatted into pages like IDMS VSAM files. The above discussion relates to IDMS VSAM files which are still formatted into pages.
To convert to IDMS VSAM database files, the DMCL needs to be changed then a BACKUP needs to be done for all the existing non-VSAM files and then a RESTORE to the new VSAM files as documented in Changing the Access Method of a File.