Hello @GaetanWalraet-7258
For this purpose, you need to use ms-DS-ConsistencyGuid as the sourceAnchor attribute for User objects. You can then switch the value of ms-DS-ConsistencyGuid attribute of old user with the new user so that a hard match can be performed with the new user account at next sync cycle. Since ObjectGUID is not configrable, you can not use it as sourceAnchor to perform hard match.
Please refer to How to enable the ConsistencyGuid feature - Existing deployment for instructions to change the sourceAnchor
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