Revisiting Incremental Learning: A Three-Interface Diagnosis of Stability and Plasticity

Incremental learning is usually evaluated through the final classifier or LM head, but this interface does not indicate where forgetting occurs. A drop in accuracy may reflect representation degradation in the backbone, readout mismatch in the head, or both. We argue that BWT and FWT should therefore be interpreted as interface-dependent quantities rather than as single model-level properties. We study three evaluation interfaces: the sequentially trained head, a newly optimized probing classifier, and a classifier-free backbone separability diagnostic based on clustering. These interfaces answer different questions: deployed performance, recoverable information under a new readout, and representation-level separability. Across CIL experiments with encoder and decoder language backbones, plus discriminative-backbone TIL sanity checks, we find that qualitative conclusions about forgetting and plasticity change substantially with the evaluation interface. Standard heads often indicate severe forgetting, probes often suggest near-complete recoverability, and backbone diagnostics suggest intermediate representation changes and a stability--plasticity pattern. This pattern is consistent across five clustering algorithms and three clustering metrics, suggesting that it is not an artifact of a single diagnostic choice. Finally, we use Just LM-Head Tuning (JLT) as a head-only intervention to quantify recoverable performance on a fixed incrementally trained backbone. The large gap between standard and head-realigned performance suggests that many failures attributed to catastrophic forgetting are better understood as head--backbone mismatch plus partial representation drift. Our results call for reporting where forgetting is measured, not only how much forgetting is observed.