Table & missing target structure for incremental conversion

What causes this issue?

The given table was already converted incrementally prior to the
upgrade. A check showed that the target structure of the table used
differs from that provided by the upgrade. This means that the
definition of the table in the ABAP Dictionary would not be consistent
with its definition in the database after the incremental conversion.
This could for example be caused by Hot Packages or add-on solutions
being applied after starting the incremental conversion.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

If the incremental conversion of the table took less than 2 hours prior
to the upgrade, the result of the ICNV for this table is simply
rejected and there is another standard conversion in phase PCON. In
this case you do not need to take any special steps.
However, if the conversion time prior to the upgrade was already longer
than 2 hours, phase PCON stops and you have to make a decision.

How to fix this error?

If phase PCON stops, you have two alternatives:
You can reject the result of the incremental conversion with
Transaction ICNV. Execute the function 'Edit->Reset conversion' and
then repeat phase PCON. The table is completely converted again in
phase PCON. If you have very large tables, this could have a negative
effect on the upgrade schedule and the system might not be productive
again in time.
If you want to use the results of ICNV and definitely do not want to
convert the table again, please contact R/3 Support immediately
(component BC-DWB-DIC-AC). It might be able to correct the problem,
avoiding another conversion of the table.
DO NOT IGNORE this message as it will cause problems in later upgrade
phasees and in production after the upgrade.

Error message extract from SAP system. Copyright SAP SE.