Hi Nimrod,
yes, high reusability versus additional regression testing is definitely a crucial point in regards to your EDI mapping approach. If you choose central mappings, you could try to put some partner-specific logic into lookups (TPM, value mappings, RFC, ..). This of course could avoid or at least minimize the regression testing of existing partners. But usually this approach doesn't work properly any more if there are more significant differences in the required partner-specific mapping logic. One option is to combine your cental mappings with pre- and post-mappings, so that you have to retest existing interfaces only if changes in the central mapping cannot be avoided. Changes or bug fixes to central mappings will always happen from time to time. Even if you use completely partner-specific, non-reusable mappings you should retest our mappings in case of release or SPS updates. In our projects we developed an automated compare tool for EDI messages and usually setup own test scenarios which allow us to trigger mass tests without major effort.In the beginning it's of course some more effort to configure these automated tests, but it's really worth the effort if it comes to large scale EDI rollouts and critical interfaces.
Regards, Holger