Hi.
What this means is that the "modified" structure of the table in your old source release is now the same as the SAP Standard Structure for the table in your target release and as such it wants to know if you wish to revert to the SAP Standard.
Doing so will mean there is no longer a modification for the table as it is now SAP Standard and will no longer be reflected in SPDD, the table structure will not change simply the modification in SPDD will no longer exist.
The SAP Recommendation is that you reset to the SAP Standard once you are happy they are the same. What is very important is that your bind the SPDD and SPAU transport requests to the upgrade, importing transports into the Shadow instance directly can result in issues with the Modifications reflected.
Thanks
Neil Ryan
SAP Active Global Support.