Is it obligatory to dispose of usurious gains acquired from commercial bank deposits, taking into account the devaluation of the currency and the unavailability of Islamic alternatives in the past? And is it sufficient to switch to dealing with Islamic banks for past transgressions to be forgiven?

A savings account in an interest-based bank is an interest-bearing loan, not a legitimate investment. The owner of the account is only entitled to their principal. They must dispose of the usurious interest by spending it on the general welfare of Muslims, because Allah Almighty says: ﴿وَإِنْ تُبْتُمْ فَلَكُمْ رُءُوسُ أَمْوَالِكُمْ لَا تَظْلِمُونَ وَلَا تُظْلَمُونَ﴾ [And if you repent, you may have your principal. Do not wrong, and you will not be wronged.] (Quran 2:279). They will not be harmed by losing this interest, as it does not belong to them. As for depositing money in an Islamic bank, it will make the transaction permissible in the future, but repentance for past interest remains obligatory.