A major question is whether ‘Hero Kim Gun-ok’ is the same submarine which was showing in 2019 when Kim Jong Un visited Sinpo. That boat, known in the West as Sinpo-C, was undergoing conversion to carry missiles. The temptation is to say that it is probably the same submarine. However, a comparison if the available imagery identifies significant and numerous differences.
If they are the same submarine, then the subsequent modifications after 2019 are extreme. It would be as if they rebuilt it twice, and also changed minor details along the way. A more straightforward explanation is that there are at least two ROMEO class submarines undergoing modification.
Sinpo-C had telltale signs that it was being modified to carry much larger ballistic missiles, like the Pukguksong family. There were vent holes in the upper sail consistent with the Gorae class test submarine, which launches these missiles. Pukguksong are strategic missiles, yet Hero Kim Gun-ok is described in North Korean sources as having a tactical relevance.
One explanation is that the two sets of conversion are aimed at building both 'strategic' and 'tactical' missile submarines. Both could however carry nuclear weapons, so the term 'tactical' is contextual.